The Rockefellers and Du Ponts in giant industries

One eats Indian sweet pastry (khoa); the other prefers French eclairs. One reads the Bhagavad-Gita, the other digs into Henry Kissinger's memoirs. One works out of his home, filled with ancient Hindu sculptures; the other works in a Victorian building decorated with European paintings.

They are Ghanashyam Das Birla and Jehangir Ratanji Dadabhoy Tata (or ''G. D.'' and ''J. R. D.'' to most), respectively, the top two partriarchs of Indian industry, and living symbols of the country's once-strong tradition of ''family business houses.''

Although poles apart in tastes and even caste, the Tatas and Birlas are all canny businessmen, leaders of India's leading corporate empires.

Yet, as both men have steadily withdrawn themselves from their paternalistic rule in recent years, they leave behind only a few relatives to command their industries and a large professional, Western-oriented cadre of managers.

The Tatas and Birlas might compare with the Rockefellers and Du Ponts in the United States decades ago, lording over colossus conglomerates. In a land where loyalty is at a premium, the practice of the ''joint family,'' or kinship in all matters, spills over into business easily.

While the Tata and Birla companies are puny by American standards of General Motors or United States Steel, they nonetheless stand out in India for each being three to four times the size in assets of other ''houses,'' such as the Modis or Singhanias.

And as they expand, the family's stake in each company lessens while the share of other stockholders rises. Still, today, about 80 of these ''houses,'' as they are called, control more than half of India's private industry.

Just how the Tatas and Birlas survive in a poor nation built on the ''socialist pattern of society'' is as fascinating to watch as a snake charmer moving with his cobra.

Both houses were well rooted before Indian independence in 1947. The octogenarian G. D. Birla, who is head of the 150-strong Birla clan, started with a Calcutta jute mill in 1918. He, like many other Indian businessmen, is a Hindu Marwari from the state of Rajasthan, which has produced most of the country's trading caste, or banyas. Today there are over 80 Birla companies, with the biggest and most modern being Gwalior Rayon, with assets over $1.8 billion and joint ventures in many countries.

Although textiles are the heart of many Birla companies, it is best known in India for its 1950s-style ambassador car, made by Hindustan Motors.

Compared with the Tatas, the Birlas are known to have had greater success in getting licenses for expansion from the government. One reason was G. D. Birla's financing and closeness to Mohandas Gandhi's independence movement. In fact, the Birlas are more political - G. D. once ran for office - and more clannish, demanding more loyalty from their executives, integrating their companies Hindu-style, and always quick to see a chance to make a profit.

Aditya Birla, favorite grandson of G. D. and chief executive of Gwalior Rayon and Indian Rayon, studied chemicial engineering at the Massachusetts Institute of Technology and plots out foreign collaborations for new ventures as well as keeping access to bureaucratic corridors. With a new professionalism, the Birlas often disagree with one another on strategy. Aditya, for instance, shows an independence from his father, B. K. Birla, who heads Century Spinning.

Birlas and Tatas are almost neck and neck in size of assets: $1.5 billion vs. profits before taxes than the Tatas have had over the last few years ($120 million vs. $130 million for the Birlas in 1980).

The Tatas' long-term strength, however, lies in their accent on more basic industries: steel, trucks, cement, and power. Nearly all their factories are cited as the most efficiently run in India. In fact, Tata trucks, built ruggedly to withstand potholed roads and constant overloading, are one of the country's best exports and have a two-year waiting list in India.

The 32 Tata companies sprang from a cotton mill built in 1874 by Jamsetji Tata, founder of the empire. The Tatas, as well as about half of the companies' directors, are Parsis, a dwindling sect following the reli gion of the ancient Persian prophet Zoroaster, who preached of salvation through good works and of the battle between wisdom and evil.

The Parsis fled to India in the 7th century from what is now Iran, and as legend has it, promised local rulers they would give to the community more than they would take from it.

Tata companies are known for providing good benefits to their workers and local communities, especially around the Tata Iron and Steel Company in Jamshedpur. Tatas introduced pensions, eight-hour working days, and dehumidified factories to India, among other things. In a poor country like India, a business can't afford not to provide basics to its workers, such as housing and schooling , J. R. D. Tata says.

In fact, when the government wanted to nationalize the steel factory in 1979, the workers fought against it and won. Despite India's protected economy, without much competition, Tata products are well known for their quality. ''A cult of excellence'' is how one observer calls it.

Mahatma Gandhi said, ''Tatas represent the spirit of adventure.'' In fact, the first Tata built India's first steel mill, first hydroelectric plant, and India's first major and still most famous hotel, the Taj in Bombay. ''Tatnager'' armored cars were widely used in World War II. And J .R. D. Tata started the nation's airline, Air India, when he flew a two-seater on mail routes between Bombay and Karachi. One of the latest Tata companies exports computer software.

J.R.D. Tata, a grandnephew of founder Jamsetji, took over the corporate empire in 1938. He says he inherited a legend. ''There's a Tata ethos recognized in India. It's quite extraordinary this influence of old man Jamsetji on following generations. You have brillant managers earning one-one hundredth what they could be earning in the US.''

In 1970, an Indian antimonopoly act aimed against the big houses caused a relatively new independence for Tata companies. The Tata family now only owns 1. 5 percent of Tata Sons, the umbrella organization. Only J. R. D.'s presence and the Tata name so far keeps the companies from acting too independently.

''It's still works as an ol' boys' club,'' a Tata-watcher says. Despite highly able managers, it appears that Ratan Tata, a nephew of J. R. D.'s adopted cousin (J. R. D. has no sons) will likely take over within a few years. A graduate of Harvard Business School, Ratan Tata has gained experience in Tata's electronics company. ''It is a great problem of continuing the Tata tradition,'' says J. R. D. ''With youngsters coming along, it will be more difficult.''