By choice or need, Americans take more part-time jobs

* Ten years ago Ethel Silverstein and her husband retired to the sunny climes of Florida and a life of leisure. Or so she thought. The problem was, after 30 years of owning her own children's store, she couldn't stand ''leading the life of Riley'' for more than a year.

Finally she found a job at a Jordan Marsh department store in Hollywood, Fla. , working 25 hours a week. The top salesperson in the men's clothing department, she recently won a statewide contest on productivity.

* Lance Brown worked at a Chrysler assembly plant in Detroit for seven years before being laid off in 1979. Since then he has had a succession of part-time jobs, making ''just enough to get by. . . .''

With his union local defunct, there is little chance he'll ever work on an auto assembly line again, since waiting lists for jobs in other plants are long. Ironically, the company he is now working for part time is preparing a closed Chrysler plant for liquidation.

* When three of her five children had departed the nest, Martha Sullivan figured it was time to ''fly the coop'' herself. For a year now she has worked part time as a secretary at a yarn and thread store in her hometown of Greenwich , Conn., and recently earned the title Manpower Secretary of the Year.

For as many reasons as there are job seekers, people are going to work part time in record numbers. A domain formerly of college students pushing ice-cream sundaes and housewives earning extra cash, part-time work has suddenly become ''respectable'' as the nature of the jobs has changed and the number of professionals working fewer than 35 hours a week has started to edge upward.

From 1974 to 1982, the number of part-time workers jumped from 11 percent of the total work force to 18 percent, according to the Bureau of Labor Statistics (BLS). Economists and labor experts say the growth is evidence of an economy in transition and changing attitudes in the workplace. Among women alone, the number of part-timers in managerial and administrative jobs increased from 974, 000 in 1970 to 1,666,000 in 1982. Much of that increase is attributed by experts to women who wanted to combine a family with career-type employment and were able to negotiate part-time status with their employers.

The rise of professionals among the ranks of part-timers may be related to changes in corporate attitudes toward part-time employment. One of the largest banks in the US, Citibank in New York, recently instituted a written policy for hiring and promoting part-time workers. A number of other companies permit job sharing, in which two people work half-time each, sharing one career.

''As the number of young people entering the work force declines over the next decade, corporations will increasingly have to cater to the experienced staff people they already have on hand, and more people may prefer part-time employment,'' says Ron Ehrenberg, a professor of economics and industrial relations at Cornell University.

Not everyone who works part time does so on a voluntary basis. Part of the record increase in part-time employment has been attributed to the recession and full-time workers whose hours are cut to match lower production needs, according to the BLS. A record 5.8 million people fell into this category last year, or about 32 percent of the entire part-time labor force.

Of those working part time voluntarily, one of the largest groups is the elderly. While the full-time employment rate among the elderly rose only 1.4 percent from 1971 to 1981, part-time employment among the elderly rose 15.7 percent over the same time period.

''They should hire more of us,'' says Ethel Silverstein. ''I'm full of pep when I get up in the morning and I'm full of pep all day. People can't believe I'm 69 for all the running around I do. You'd better believe older workers can produce.''

More and more corporations apparently are agreeing with Mrs. Silverstein. Travelers Corporation, Control Data Corporation, Chase Manhattan Bank, and others are actively seeking out elderly employees. Some corporations have even set up special employment offices just for older workers.

Not all corporations that allow for part-time positions do so simply to satisfy the demands of workers who want to devote more time to their families or pursue outside interests. Companies can often avoid paying for costly extras like health insurance and retirement benefits by hiring workers part time.

For those same reasons, unions have voiced complaints about the growing emphasis on part-time positions. In 1981 the Transport Workers Union turned down a proposal by American Airlines to use part-time workers, claiming the plan would jeopardize the standing of full-time employees.

While the nature of part-time jobs is changing, many of them are still ''dead-end'' positions, often they are temporary, and a certain stigma continues to attach itself to part-time work.

''Working less than full time strikes many managers as not very professional, '' says Diane Rothberg, president of the Association of Part-Time Professionals. ''That's going to be a difficult attitude to change.''