The Supreme Court, an institution with the confident aura of one who does not apologize for existing, lays down the law about matters of life and death (abortion, capital punishment). This week it dealt with questions arising when death threatens to closely follow the beginning of life.

The ''Baby Doe'' case was one of statutory construction, not constitutional interpretation. The Rehabilitation Act of 1973 says no program, such as a hospital, receiving federal funds can deny participation to, or discriminate against, any ''otherwise qualified handicapped individual'' solely because of the individual's handicap.

In 1982, in Indiana, parents of an infant with Down's syndrome (a congenital defect involving varying degrees of mental retardation and some physical abnormalities) refused consent to routine surgery to unblock the infant's esophagus. The blockage prevented oral feeding. The child starved to death.

In response to controversy about that case and ample evidence of other children's deaths brought on by denial of normal care (care routinely given to normal infants), the secretary of health and human services in 1984 issued regulations under the Rehabilitation Act. They required hospitals to post notices that care could not be denied to infants because of mental or physical impairments, required state child-protection agencies to protect handicapped children from ''unlawful medical neglect,'' and required immediate access to a patient's records.

The American Hospital Association, American Medical Association and similar groups got a lower court to declare the regulations an invalid application of the Rehabilitation Act. The Supreme Court has now affirmed that invalidity, 5-3.

In his opinion for a court plurality, Justice Stevens (joined by Marshall, Blackmun and Powell; Burger concurred in the judgment, not the reasoning) noted that the preamble to the HHS regulations stipulated that when ''a non-treatment decision, no matter how discriminatory, is made by parents, rather than by the hospital,'' the Rehabilitation Act does not require the hospital to override the parents and provide treatment. And HHS did not show that hospitals, the recipients of federal funds, have been refusing treatment that parents have requested or courts have ordered.

Stevens said HHS equated the withholding of consent by parents with denial of treatment by hospitals. Indeed, hospitals could be sued for performing surgery without parental consent. The court's plurality held that the HHS regulations were not grounded in need because there is no evidence of hospitals' denying care ''on the basis of handicap,'' only on the basis of parental non-consent.

In dissent, Justice White was joined by Brennan, and by O'Connor in part. White correctly said the plurality was too sweeping in rejecting HHS's right to regulate under the Rehabilitation Act. The question, White said, is whether HHS has ''any authority at all'' under the Rehabilitation Act to regulate medical-care decisions concerning handicapped newborns. White said, reasonably: yes.

White said that no one, including the court's plurality, disputes what the data demonstrate: decisions are often made to deny treatment to handicapped newborns. A survey of pediatricians showed that most would acquiesce in parents' decisions to administer lethal neglect against Down's syndrome infants with life-threatening physical problems, and a significant percentage of the physicians would encourage parental nonconsent. The surveyed pediatricians said they would not acquiesce in parental decisions not to treat children with the same physical problems but who were not destined to be mentally retarded.

Such decisions are discriminatory because the treatment withheld would be automatically extended to nonhandicapped newborns. Were it not extended, many doctors would seek a court order to extend it.

White says there can be a regulatory role reasonably related to the aims of the Rehabilitation Act. The regulations would recognize that parental decisions about care for handicapped newborns -- decisions made when parents are exhausted and distraught -- are influenced by doctors and hospitals. Legitimate regulations could require recipients of federal funds, such as hospitals, to cooperate in alerting nurses and others, such as state child-protection agencies, to their responsibility not to act in a discriminatory manner regarding treatment of life-threatening problems.

Children are not chattel, but parents have de facto discretion to kill handicapped newborns through aggressive neglect. White's point is that the Rehabilitation Act authorizes a regulatory role in pulling health-care professionals and state authorities up to higher vigilance regarding professional duties and the application of child abuse and other laws relevant to such neglect.

The court was ruling on a narrow question of administrative proprieties under a particular statute, rather than the protections implicit in, say, constitutional language such as the guarantee of ''equal protection'' of the laws. Thus the terms of this case miniaturized the issue.

But the issue is, daily, a matter of life and death in hospital conference rooms. Decisions taken there are shaped by the nation's moral sensibilities, and shaping those is a proper goal for federal action.