Come to the World's Fair - and see New Orleans.

World's Fairs - even at their self-propagandizing best - aren't what they used to be. I base that on spending three days each at the New York's World Fair in 1964- 65, Montreal's Expo 67, and the current Louisiana World Exposition, in place here through Nov. 12.

If those technological expositions in the '60s define the standard, this latest one is not in the same category. It's simply a misnomer (as is the word ''pavilion,'' which even some of the most minor attractions are labeled).

Yet the comparison is a little unfair. Even at 82 acres, $350 million spent, and 23 countries taking part, the Louisiana fair doesn't have the size, scope, money, commitment, or attraction of those fairs of old. Fifty million people saw Expo 67 and its pavilions from 70 countries in the first year. Only 12 million, on the other hand, are needed to break even here, and at recent attendance figures they'll get only half that - unless the record-breaking crowds of Memorial Day are a harbinger of the future.

But before I continue, I've got to admit there are just enough things I got a real kick out of to make this fair worth seeing on its own merits - as if you needed another reason to visit wonderful New Orleans.

Tops on the list are the stunning 3-D movie on freshwater lakes and rivers at the US pavilion, or the equally stunning film on the same subject shown on the enormous screen at Canada's pavilion. The official theme, ''The World of Rivers: Fresh Water as a Source of Life,'' is a worthy one, and an issue I learned much about after playing the gadgets and computers at the many hands-on exhibits. (''Only 3 percent of the earth's water is fresh, half of that undrinkable,'' for instance. Or ''Canada has more fresh water than the rest of the world combined.'')

I also had some great Cajun catfish, crawfish, spicy fried chicken, creole jambalaya and potato cakes. The 370-foot gondola ride over the mighty Mississippi sent chills up my spine. The boat ride through makeshift sets of Louisiana plantations was sweet, especially through kids' eyes. The Aquacade - replete with high diving boards - was a kick, too, even though the 15 Olympic divers didn't hit the water at the same time in their spectacular finale, as they said they would.

But something must be said about the considerable hype that permeates these affairs - upon which they thrive or die. I've never forgotten the Life magazine ad I read as a youngster that said, ''Experience weightlessness at the USSR pavilion at Expo 67. How the Russians do it is their secret.'' After a year of arrangements and much ado, I finally sat down to a Russian space-program propaganda film whose theater seats briefly rotated to simulate weightlessness.

Similarly, and sometimes more subtly, the exhibits, movies, and slide shows here seek self-justification for the host industry or country.

Inside the Petroleum Industry's gigantic oil drilling platform, for instance, you hear a tape that explains how drilling rigs are good because they provide underwater organisms with a habitat for lodging and breeding. (Nothing is said about environmentalists' real fears: drilling mud that seriously threatens offshore fish breeding.) How does offshore oil drilling fit the exposition's theme of rivers and fresh water? The viewer is left to guess by the title: ''Petroleum Industry: Where Oil and Water Meet.'' And what does the history of Lipton tea - as one exhibit touts - have to do with rivers?

In a relief map at the Italian exhibit you can follow the bulb-lit route of waste water around the Bay of Naples, then read about the superiority of the Italian school of hydraulics. As you leave, there is a collage of Italian-brand mineral waters. (The French exhibit next door explains the superiority of Perrier.)

UNICEF explains dehydration and diarrhea in third-world countries. Soft Sheen Products, official hair-care sponsor of the fair, sponsors the Afro-American pavilion, featuring a historical overview of black cosmetology. The list is endless. Such is the stuff expositions are made of.

So forgive a few caveats for those moms and dads who are going to pile the kids in the back of the station wagon and drive 600 miles, as my family did in the '60s.

One saving grace of the site itself is its location between downtown high-rises and the river, whose breezes keep the temperature bearable. It's just a short walk from some major hotels (notably, the Hilton and the Sheraton). The walk is not a pedestrian's paradise; a shuttle service is provided for most hotels in the area. Eleven hotels have been built over the last two years, so Mayor Ernest (Dutch) Morial guarantees a room for everyone. (The argument has been passed around, however, that the advent of television has obviated the need for world expositions, because both exhibitors and hosts can expound their wares and messages better via the tube.)

I'd recommend staying in the French Quarter - about a 15-minute walk from the grounds - to take in the wonderful night life and ubiquitous antique shops. The Mobil Travel Guide gives a good list of where to stay.

The ubiquitous jazz, Dixieland, and country Cajun music were wonderful, as were clarinetist Pete Fountain and trumpeter Al Hirt, each with his own theme restaurant. But on the international side, I'm sorry if I don't think sending a few native carvings and grass chairs, or nailing some bamboo over two-by-fours, constitutes a pavilion.

Did fair organizers count the ''African'' marketplace or ''Inca Products From Bolivia'' on their list of 23 participating nations? And does building some makeshift facades for Greece, Germany, Italy, Belgium, etc., and then doling out fast-food versions of souvlaki, wurst, lasagna, and Belgian waffles constitute a feast of international cuisine? Typical of World's Fairs, perhaps, but this one was touted as a paradise for the palate.

Similarly - and in comparison with those earlier fairs - a 1.5-mile monorail and the redevelopment of a 100-year-old warehouse district do not a World's Fair make. The site seems visually cluttered, even at the so-called center, where a large pool with fountains is surrounded by mammoth composition alligators scaling the ''Wonderwall'' - at this point made up almost solely of interlocking pipes. One critic called this an urban-renewal project masquerading as an exposition. Since the fair is the catalyst for projects amounting to $2.6 billion for New Orleans development, he has a strong case.

Although the fair is touted as a ''world'' exposition, Louisiana Gov. Edwin Edwards may have told the crowd a little more of the reason behind wanting to host such a fair: ''We've been waiting 100 years to show the world what Louisiana has'' (referring to Louisiana's last exposition here, in 1884). And Mayor Morial said it was a way to show off New Orleans pride and people.

On purely consumer considerations, fair organizers have gone a long way to provide comfort. Fountains and water are everywhere for fresh atmosphere, lined with omnipresent benches. Walkways are covered by black nylon mesh to keep off the hot sun. Electric tricycles are provided for the handicapped - and the gondola makes special stops to accommodate them.

One reporter found the real reason behind building the 2,400-foot-long meandering Wonderwall - a three-story-high, 10-foot-wide collection of gazebos, columns, busts, cupids, domes, chimneys, and animal sculptures. The purpose was to disguise low-hanging power lines instead of its purported reason: tying these chaotic grounds together with a single ''Mardi Gras'' theme.

There is a Mardi Gras flavor, however, with music easily the fair's biggest asset. At any given time a host of live bands are playing the midway, various exhibits, or the Jazz & Gospel tent.

The entrance fee (recently cut from $14 to $9.95) is certainly worth it. And all pavilions except the Vatican's ($5 - the art is worth it, by the way) are free (as is the monorail, Aquacade, jazz tent, American Showcase Theater, and Louisiana Folklife Festival). But the Times-Picayune/States-Item newspaper says a family of four will spend more than $100 - after the price of admission.

Although I spent $3.87 for a small slice of pizza and Coke, it is said no entree goes for more than $6.50. Official guidebooks cost $7 and a map of the grounds costs $3. The gondola costs $3.50. A new after-10 p.m. rate ($4) has just been introduced; the pavilions are closed then, but restaurants are still going strong.

The organizers originally intended to have a fair you could see in eight hours. By opening day, they said it would take 24 hours. It took me three 10 -hour days of hard fair-going to see everything.

Based on that, I would never tell someone to make a special trip - especially a long distance - to see just the fair. But I would say it's certainly an added reason to see New Orleans with its wonderful French Quarter and Bourbon Street jazz.