The Supreme Court declined to enter the contentious field of gay parental rights yesterday, rejecting an appeal of a groundbreaking Massachusetts court decision that declared a lesbian woman the "de facto" parent of her former partner's son.

The case arose out of a custody dispute in Massachusetts, where in June the state Supreme Judicial Court upheld temporary visitation rights for the woman who had been raising the child with his biological mother until the couple's break-up last year.

The pair, known in the court briefs only as L.M.M. and E.N.O., had been living together in Maryland when L.M.M. became pregnant through artificial insemination in 1994. They sent out birth announcements naming them both as parents, and shared child care duties. According to E.N.O's filing, the baby called them "Mommy" and "Mama," respectively.

In 1997, the couple moved to Massachusetts, where their relationship began to deteriorate and the custody battle over the boy, who is now almost 5, started.

The Massachusetts court decision, which describes E.N.O. as a "de facto," or "in fact" parent, is part of a new body of law emerging as state courts grapple with domestic disputes involving lesbians and gay men. Decisions have varied over whether ex-partners are entitled to visitation rights. In the Massachusetts case, the majority said, "It is to be expected that children of nontraditional families, like other children, form parent relationships with both parents, whether those parents are legal or de facto."

In her petition to the Supreme Court, L.M.M. contended the Massachusetts court had lacked the authority to hear the claim by E.N.O., who had never adopted the child, and that the ruling had infringed upon L.M.M.'s fundamental right to raise her child without state intrusion.

The justices refused to hear the case of L.M.M. v. E.N.O. without comment or recorded vote, and their decision sets no national precedent. The battle between the two women over custody and permanent visitation rights is ongoing in Massachusetts courts.

Separately, the Supreme Court agreed yesterday to decide whether a federal law making it a crime to commit arson on property used in interstate commerce may be applied to private homes. Federal appeals courts are in conflict over whether Congress had constitutional authority to punish arsons on residences, and the case of Jones v. United States will offer the justices yet another chance to consider the breadth of federal power to regulate interstate commerce. In recent cases, the court has repeatedly ruled Congress reached too far into state affairs.

Finally, the justices spurned Juan Raul Garza's challenge to his death sentence for three Texas murders earlier this decade. One of 20 men on the new federal death row near Terre Haute, Ind., Garza was convicted of drug smuggling and murder under a drug-kingpin law requiring capital punishment for murder arising from trafficking. Garza's case is far enough through the appeals process that he could become the first person executed by the federal government since 1963. The justices rejected the case of Garza v. United States without comment.