The day-old Brown infant, the first child ever conceived in a glass dish and successfully carried to term, left the premature unit at Oldham General Hospital yesterday to lie in the arms of her mother. Both were said to be "doing well."

Gynecologist Patrick Steptoe and physiologist Robert Edwards, the tired but triumphant inventors of the new technique, confidently declared that many other infertile women will profit from their work. Steptoe insisted that the implantation of an embryo in the womb of Lesley Brown was no lucky fluke but will lead to "many successes."

He and his partner, appearing at a press conference in a Manchester hospital near the Oldham birth place, were still guarded, about the details of their triumph. But Steptoe did disclose that his technique for extracting an egg from Mrs. Brown involved only a "minor operation," one that could be completed in 8.5 seconds.

Many details about the baby's conception and birth are still unknown. The London Daily Mail syndicate bought exclusive rights to the story for a reputed $570,000.

Because of the terms of that contract, Lesley Brown has been visited only by her doctors, her husband, John, a 38-year-old truck driver, and Daily Mail reporters. Yesterday she was quoted as saying of Steptoe:

"He was marvelous. We will never be able to thank him enough. Having this baby is the most incredible thing in the world."

In its copyright story, the Mail quoted John Brown as saying:

"She's beautiful, beautiful. She a beautiful little girl . . . Our happiness is complete . . . She was bawling her head off, just like me . . . I am not a religious man but I thank God that I heard our little girl cry for the first time. No one can realize what this means to Lesley and myself."

At their press conference, Steptoe and Edwards offered a few fresh glimses into their work. Steptoe explained he had chosen Mrs. Brown because "she was in an age group that was highly suitable . . . not too old . . . highly fertile."

He noted that she had endured many operations to unblock her fallopian tubes, a common history among patients who consult him and one that causes problems. "We must have ready access to the ovary," he said.

Steptoe stimated that he had tried implanting an embryo fertilized outside its mother's womb about 200 times before the succeeded with Mrs. Brown.

He said he had delivered the baby by Caesarean section nine days before it was due because he had detected a slight toxemia in Mrs. Brown that could have led to a stillbirth. He suggested that the stress of press attention may have brought on this condition. So far as is known, however, only the Daily Mail, with whom Steptoe negotiated, had been allowed to see his patient.

Mrs. Brown will never conceive in any other way, Steptoe disclosed. Last year, in a preparatory operation, he removed both her fallopian tubes.

The procedure, Steptoe said, could also help men whose low sperm count made conception difficult. Only a few sperm are needed, he explained, to fertilize an egg in a glass dish.

The gynecologist predicted that "within a fairly short time, a large number of teams" will learn the technique he and Edwards used and "there will be many successes."

Edwards, who teaches at Cambridge, backed up his partner, saying:

"We are very likely to make this a successful operation and get many more babies from this technique . . . I've no doubt we will do it again and again and it will become an established procedure."

Edwards suggested that it could be used to solve other problems of infertility, unlike those of Mrs. Brown.

The pair hinted, but declined to say flatly, that they have implanted other women who are now carrying embryos created outside the womb.

In over-simple form, Steptoe extracted an egg from Mrs. Brown's uterus. Edwards then fertilized it with her husband's sperm in a nutriant placed in a glass dish. When the embryo divided into several cells after a few days, Steptoe reinserted it into its mother's womb.

Edwards remarked that the last time he had seen the baby, whose name has not been discussed, "she was a beautiful eight-celled embryo."

Steptoe said, "One is very pleased and very relieved that the anxieties are over. We've got a nice, healthy normal baby."

The two doctors made a strong pitch for bigger and better laboratories. Steptoe said his facilities at Oldham were "inadequate for further progress." He hoped a larger work place would be built in Cambridge. Edwards figured he had traveled 500,000 miles from Cambridge to Oldham and back to work with Steptoe.

To dramatize their work, the pair allowed the government's Central Office of Information to film the birth of the baby. Sequences will be shown and are expected to be an excellent fund raiser.

Other doctors, of medicine and theology, are already weighing in with their comments on the birth of the 5-pound, 12-ounce girl.

Prof. Sir John Dewhurst, president of the Royal College of Obstetricians and Gynecologists, said "it is an exciting piece of original research and it is very hopeful for certain childless women in the future."

Gordon Cardinal Gray, archbishop of St. Andrews and Edinburgh and president of the Scottish bishops, however, said, "I have gave misgivings about the methods and possible implications for the future. The church holds that a child should be the product of a loving union between husband and wife."

Another Catholic cleric here took a less severe view. Bishop Augustine Harris, president of the social welfare commission of the Catholic bishop's conference, said:

"Some married couples have a deep desire for children but are unable to conceive. Science can support the loving and natural ambitions of the couple to produce new life."

Newspapers are also having ethical and economic problems over whether to buy the Daily Mail package. A leading Italian newspaper was offered the rights for 10,000 pounds - about $19,000 dollars - but declined on the grounds that any such sale was a crude exploitation.

The health of the baby is of more than scientific and sentimental concern. Every day she lives compounds her values to the Daily Mail syndicate.

A paper that purchases the Mail's package of text and pictures get a 40 percent discount if the baby fails to live a week. If she survives less than 28 days, the Mail will negotiate an appropriate discount with its customers. Thereafter, the syndicate's clients will enjoy the Mail's exclusive rights for the first 400 days of the baby's life.

Whether the Browns and their doctors must forfeit any portion of their share of the Mail payment in the event of an early death is not known.