As attorney general, he took action on his own and announced in November 2001 that doctors who prescribed lethal doses of drugs for the purpose of assisting a suicide risked losing their federal licenses to prescribe all "controlled substances." Without such a license, known formally as a "registration," doctors would find it very difficult to practice medicine, because controlled substances include many prescription pain relievers and other commonly used medications.

Oregon sued to block enforcement of the new rule, joined by a doctor, a pharmacist and several terminally ill patients.

As authority for his action, the attorney general invoked the Controlled Substances Act, a 1970 law that established the framework for federal drug policy, as well as an implementing regulation issued by the Justice Department the following year. The regulation requires that every prescription for a controlled substance "be issued for a legitimate medical purpose." Assisted suicide is "not a legitimate medical purpose," Mr. Ashcroft declared in what he called an "interpretive rule."

In his opinion today, Gonzales v. Oregon, No. 04-623, Justice Kennedy went out of his way to emphasize, with evident disapproval, the unilateral nature of Mr. Ashcroft's action, taken "without consulting Oregon or apparently anyone outside his department." The attorney general's rule was not entitled to the deference the court usually gives to interpretations by executive branch officials of their governing statutes, he said, because Congress had not given the attorney general the authority he was invoking.

"The authority claimed by the attorney general is both beyond his expertise and incongruous with the statutory purposes and design," Justice Kennedy said.

Justice Scalia, in his dissenting opinion, took issue with the argument that Congress could not have intended to delegate medical judgments of this sort to the attorney general. The legitimacy of physician-assisted suicide "ultimately rests, not on 'science' or 'medicine,' but on a naked value judgment," he said, adding: "It no more depends upon a 'quintessentially medical judgment' than does the legitimacy of polygamy or eugenic infanticide."

In any event, Justice Scalia said, Mr. Ashcroft's action was "the most reasonable interpretation" of the statute and the regulation because "virtually every relevant source of authoritative meaning confirms that the phrase 'legitimate medical purpose' does not include intentionally assisting suicide."