Responding to a scientific study showing that a large percentage of bottlefed babies in Brazil's largest city are undernourished, the conservative military government of Brazil has launched a campaign to encourage mothers to breast-feed their children.

The campaign was yet another blow to Nestle, the Swiss-based multinational corporation that has been under fire for several years for promoting the use of infant formula in underdeveloped countries. Nestle, the sole commercial supplier of infant formula in Brazil, has been accused of encouraging poor mothers to use its formula even though the women often dilute it with unsafe water or are unable to prepare it under santiary conditions.

The worldwide campaign against Nestle' may even become an issue in the debate over confirmation of President Reagan's nominee for assistant secretary of state for human rights, Earnest W. Lefever, if the Ethics and Public Policy Center in Washington. The center, which reprinted a Fortune magazine article defending Nestle and describing its church-supported opponents as "Marxists marching under the banner of Christ," has accepted a $25,000 contribution from Nestle.

Brazil's campaign to promote breast-feeding, launched on the 17th anniversary of the anticommunist coup that brought the military to power here, is undoaubtedly the Third World's largest such campaign.

Bearing such slogans as "Infant mortality is five times lower among breast-fed babies," millions of posters of nursing infants are being tacked up in school rooms, health clinics and maternity wards. Soap opera idols have been mobilized for a blitz of radio and television advertising. Starting this month, all light, gas, water and telephone bills -- even betting cards for the immensely popular national soccer lottery -- will carry the slogan: "Nurse your child."

"I tell mothers their milk is for their babies -- cow's milk is good for calves," Dr. Yedda Paschoal de Oliveira, director of the nationwide drive, commented when interviewed recently in her Brasilia office.

Dr. Paschoal calculated that last year families in Brazil spent $100 million on powdered milk, which she said meant 180 million quarts of mother's milk was wasted.

Although breast-feeding was virtually universal in Brazil up until the zilian babies are nursed past two months. By six months, the proportion has dwindled to 2 percent.

Reaching under a stack of breast-feeding posters, Paschoal pulled out a study by the Sao Paulo School of Medicine that is the backbone of the campaign in Brazil, and has implications for other Third World countries. Working in 1979, the researchers closely monitored the feeding and growth of 191 babies in low income families in Sao Paulo.

The report found that:

32 percent of the bottle-fed babies suffered from malnutrition, compared to 9 percent of the breast-fed babies. Experts here say 70 percent of the weight of the brain is formed during the first two years of life, and malnutrition at this age often causes irreversible mental deficiencies.

32 percent of the bottle-fed babies had to be hospitalized, while none of the breast-fed babies had to be hospitalized.

Most of the hospitalized babies had diarrhea, which is the most frequent cause of infant death in Sao Paulo.

A separate study of infants from six to 11 months in Sigulem, a Sao Paulo suburb, found that 96 percent of deaths registered were among children breast-fed less than six months.

A Nestle spokesman in Sao Paulo said recently that he was not aware of the School of Medicine research.

But the survey findings were echoed in random interviews with mothers in Rochinha, one of Rio's largest -- and worst -- favelas or shantytowns.

"I started giving Nestogeno [formula] to my baby boy at 1 1/2 months, but he got a stomach infection that almost killed the child," one woman, known only as Kichinha, said as she sat in her two-room house, partially roofed with flattened soy oil cans.

Nicina said her baby was consuming four cans of formula a week. To save money she diluted the mix with flour, corn meal and cream of rice cereal. At $2.50 a can, a month's supply of Nestogeno costs $40, while most wage earners of Rocinha earn Brazil's minimum salary -- $80 a month. The Sao Paulo study found that formula feeding a baby for one year would require 43 percent of the income of a poor family of four, compared to the cost of breast-feeding, approximately 4 percent.

"The water here also doesn't help," Nicinha said, waving to a green-gray open sewer gurgling three feet from her doorstep. Favela residents get their water from nearby wells, but they say this water is polluted, frustrating efforts to sterilize baby bottles.

Across the sewer and down a narrow, slippery, narrow alley Maria da Conceicao sat in her darkened room, nursing her 1-year-old son, Vito.

"They are much stronger," she said of breast-fed babies. "They don't get sick with diseases."

Dr. Amandio Ferreira de Souza, head pediatrician at Rio's Poor Mother Hospital agreed with Maria da Conceicao's comments.

"Almost all the diarrhea cases I see are bottle-fed babies," he commented.

Without hesitation, Dr. Amandio said his fellow pediatricians in Rio are "100 percent" in favor of breast-feeding, but for many years his profession was the target of a multifaceted promotional campaign by Nestle.

Until recently, Nestle gave a free, one year's supply of powdered milk to Brazilian pediatricians, nutritionists and maternity nurses when they or their wives had children.The Sao Paulo report found that Nestle advertising helps support professional journals, and Dr. Amandio said the company frequently sponsors meetings of pediatrians.

Nestle spokesmen maintain that the company never advertised their infant formulas to the general public, but critics charge that promotion occurs in subtle forms.

"The cans always say, 'Mother's milk is best for your child, but in the absence of milk . . .'" commented dr. Paschoal, director of the nationwide nursing drive. "It is that 'but' that creates doubt in the mother's mind. Studies show that doubt and worry cause the milk to dry up."

In another case, the February issue of Cladia, Brazil's largest circulating women's magazine, carried an unsigned "debate" on breast-feeding. The article contrasted "mothers of the most saving tribes of Africa [who carry] their babies slung around their necks" to "professional, active, independent, women [for whom] breast-feeding can constitute a prison." Sandwiched among the pages of the "debate" was a full-page advertisement for a Nastle baby cereal.

"Claudia's" readers are middle and upper class, and such advertising has been very successful, according to Ines das Gracas Maria, head nurse of the nursery at Santa Lucia private hospital, located in Rio's affluent southern zone.

"The mothers here come up with any possible excuse to get out of breast-feeding: their breasts hurt, they don't have milk, their breasts will fall -- sometimes they almost come to blows with the pediatricians over it," she said.