ASHINGTON, Dec. 2 — The police need not wait more than 15 or 20 seconds after knocking before they break down a door to search a residence for drugs, the Supreme Court ruled unanimously on Tuesday.

The decision overturned a ruling by the United States Court of Appeals for the Ninth Circuit, in San Francisco, which invalidated the search of a Las Vegas drug dealer's apartment on the ground that the brief interval between knocking and forcibly entering was constitutionally unreasonable.

Justice David H. Souter said Tuesday that the reasonableness of a forced entry depended on the circumstances of each case, rather than on a particular formula or fixed time limit. In this case, he said, the Las Vegas police, who had a search warrant and were assisted by federal agents, knew that the suspect, LaShawn L. Banks, was likely to have drugs in his apartment.

"We think that after 15 or 20 seconds without a response, police could fairly suspect that cocaine would be gone if they were reticent any longer," Justice Souter said.

The decision added some content to recent Supreme Court rulings that referred in only general terms to the requirement that the police make their presence known before entering a residence to execute a search warrant. The question was "how to go about applying the standard of reasonableness to the length of time police with a warrant must wait before entering without permission after knocking and announcing their intent," Justice Souter said.

The answer was not one likely to provide much comfort to criminal suspects. Mr. Banks argued that he had not refused to open the door, but rather was in the shower and did not hear the police, making the officers' rapid entry all the more unreasonable in his view.

But the fact of the shower was irrelevant, Justice Souter said. "As for the shower, it is enough to say that the facts known to the police are what count in judging reasonable waiting time," he said.

Most important, the decision said, was the fact that this was a drug search, and that drugs can disappear within seconds down a sink or toilet. "It is imminent disposal, not travel time to the entrance, that governs when the police may reasonably enter," Justice Souter said.

The decision, United States v. Banks, No. 02-473, was not a complete victory for the government. The solicitor general's office had argued that whether the officers caused property damage during their entry was irrelevant to the question of whether the entry was reasonable. But the "totality of the circumstances," including the fragility of the evidence being sought, should always be considered, Justice Souter said, adding, "Police seeking a stolen piano may be able to spend more time to make sure they really need the battering ram."

In a second decision on Tuesday, a case under the Americans With Disabilities Act fell short of providing an answer to the question the court had expected to decide: to what extent does the law require employers to bend neutral rules of the workplace to accommodate employees with disabilities?

The employee in this case, Raytheon Company v. Hernandez, No. 02-749, was a former drug user who had been dismissed because of his drug use and who sought re-employment after his rehabilitation. Like many companies, Raytheon has a flat rule against rehiring anyone who was dismissed for workplace misconduct. Because the disability act protects recovering drug users, the Ninth Circuit ruled that the no-rehire policy violated the law because it had the effect of screening out people with a record of drug addiction.

The Supreme Court vacated that decision in an opinion by Justice Clarence Thomas, who said that a neutral policy such as Raytheon's could not, on its face, be said to be discriminatory. Justice Thomas said the real question was whether the company improperly based its refusal to rehire the former employee, Joel Hernandez, on the basis of his disability rather than on a policy that applied to all, a question that the appeals court's decision did not address.

Condon McGlothlen, an employment law specialist at the Seyfarth Shaw firm, said employers had been watching this case for guidance on what types of accommodations the law requires on behalf of workers with disabilities, guidance that he said will have to await another case.

The justices ruled 7 to 0, with Justices Souter and Stephen G. Breyer not participating, for reasons they did not specify.