An Indiana couple, backed by the state's highest court and the family's doctor, allowed their severely retarded newborn baby to die late Thursday night, marking a new chapter in the emotional debate over the right to end someone else's life.

The baby boy, known to the public only as "Infant Doe," died six days after he was born with Down's syndrome, or mongolism, and other respiratory and digestive complications requiring major surgery. The condition led his parents to the decision to withhold food and treatment and, as the parents' lawyer put it, "let nature take its course."

Death came as Indiana authorities rushed to Washington seeking Supreme Court intervention to override lower court decisions giving the parents the right to make the choice.

Doctors say that similar decisions are made periodically in major hospitals all over the country. But they are usually private, between parents and physician, without publicity and without involvement by the courts.

The events in Bloomington, Ind., were different because they were so public and because the courts sanctioned the death of a newborn child who, with surgery, might have lived.

He had a 50-50 chance of survival, albeit with a grave burden of retardation, according to the parents' lawyer, Andrew C. Mallor. But he maintained that it was a "private matter."

"This case never should have been brought into the judicial system," Mallor said. "We have no way of dealing with it . . . . "

Following the dramatic events from the birth of the baby to its death Thursday night, legal, medical and ethical experts yesterday examined the case in the light of a decade of still unsettled thinking about how to handle these situations. "Right-to-life" activists, at the same time, vented their rage that a child had been allowed to die.

The details of the child's condition at birth and the medical considerations that entered into the parents' decision remained sketchy, in part because the identities of the parents and their doctors are secret under Indiana law and Bloomington Hospital officials refuse to answer specific questions.

An autopsy report released yesterday afternoon by the Monroe County coroner said only that a preliminary examination confirmed that the child had Down's syndrome with "tracheoesophageal fistula," a rare complication in the respiratory tract.

The complication makes it impossible to eat normally because the esophagus, or swallowing pipe, is improperly formed, generally with a blockage that does not allow food to reach the stomach, and also has a linkage with the trachea, or breathing tube. The defect can often be corrected through surgery.

Mallor provided few details about the unnamed parents. The mother is 29, he said, and was thought too young to undergo amniocentesis, a test that can be used during pregnancy to reveal genetic problems. Mallor said the mother and father were trained as teachers and have taught handicapped and retarded children, though the father is now a "management executive" and the mother a "housewife" caring for two healthy children.

At the center of the troubling case was uncertainty about the infant's chances of survival, with or without surgery. Down's syndrome is a genetic condition that occurs, by some estimates, roughly once in every 700 to 900 live births and is more frequent in older mothers. It is generally characterized by varying degrees of mental retardation and facial abnormalities.

While a child with this condition may survive to young adulthood or even old age, life expectancy is often decreased by heart disease and other complications.

In the case of "Infant Doe," the Down's syndrome accompanied by the rare abnormality in the breathing and swallowing tubes increased the likelihood of accompanying heart problems.

The parents received two medical opinions, Mallor said. "The child could have surgery with an even chance of success, major, multiple surgery. Nothing could be done to treat" the retardation, he said.

Second, "they were told they could let nature take its course, that the child could contract penumonia and die.

"They were told this would be the alternative to having a child with severe handicaps."

Mallor and Barry Brown, the Monroe County prosecutor, said it was the hospital that went to court over "Infant Doe" in an attempt to protect itself from possible lawsuits or criminal charges stemming from the baby's death.

Brown said he then learned of the case and sought a court order removing the infant from his parents' custody and allowing necessary medical treatment to keep him alive.

The court battle ended at the Indiana Supreme Court, which declined to review the lower court ruling not to intervene with the parents' decision.

Judith Areen, a Georgetown University Law Center professor familiar with the field, said the state Supreme Court's action was a departure in that "generally speaking the courts have ordered treatment" in similar cases.

Lawyers from the prosecutor's office and a court-appointed attorney representing the infant left for Washington, D.C., Thursday evening to seek intervention by Supreme Court Justice John Paul Stevens, who handles emergency orders for the Indiana area.

Food and water were withheld from the infant from the beginning, subjecting it to the life-threatening combination of dehydration and starvation. The autopsy report did not determine the exact cause of death "pending further studies which should be completed within the next week." Mallor said he understood that the baby had died of asphyxiation stemming from pneumonia.

Medical experts indicated that if intravenous feeding had been continued, without surgery, the baby could have survived for a period of time, but would likely have succumbed to pneumonia induced by acids from the stomach reaching the lungs.

"We've dealt with many other problems of treatment stopping that are pretty well settled," said Robert M. Veatch, professor of medical ethics at the Kennedy Institute of Ethics here. "This particular set of circumstances is in my view the most difficult and we'll be struggling with it for a while.

"The two critical variables are the extent to which the child could be said to be inevitably terminal. The closer you get to that the easier it is to justify the decisions."

"These choices are being made in hospitals every day," said Alexander Capron, executive director of the President's Commission for the Study of Ethical Problems in Medicine. "It often depends on the orientation of physicians. Some lean heavily on the parents to have the treatment. Others believe parents should make the decision."

"Right to Life" organizations expressed outrage yesterday, noting that several people had made public offers to take custody of the infant. "I think it's really disgusting," said Janet Rebone, president of Monroe County, Ind., "Right to Life."

"The mother was there almost all day yesterday," said Mallor, the parents' lawyer, "just as she has been there with the child since the beginning. It wasn't a case of abandonment. It was a case of love."