More than 60 years ago, the Supreme Court ruled that the police were not entitled to enter a residence without a warrant merely because they smelled burning opium.

On Wednesday, at the argument of a case about what the police were entitled to do on smelling marijuana outside a Kentucky apartment, two justices voiced concerns that the court may be poised to eviscerate the older ruling.

"Aren't we just simply saying they can just walk in whenever they smell marijuana, whenever they think there's drugs on the other side?" Justice Sonia Sotomayor said, considering what a decision against the defendant would signal to the police. "Why do we even bother giving them a warrant?"

The old ruling, Johnson v. United States in 1948, involved the search of a hotel room in Seattle. The smell of drugs could provide probable cause for a warrant, Justice Robert H. Jackson wrote for the majority, but it did not entitle the police to enter without one.

"No suspect was fleeing or likely to take flight," Justice Jackson wrote. "The search was of permanent premises, not of a movable vehicle. No evidence or contraband was threatened with removal or destruction."

In the new case, police officers in Kentucky were looking for a suspect who had sold cocaine to an informant. They smelled burning marijuana coming from an apartment, knocked loudly and announced themselves.

Then they heard sounds from inside the apartment that they said made them fear evidence was being destroyed. They kicked the door in and found marijuana and cocaine but not the original suspect, who was in a different apartment.

The Kentucky Supreme Court suppressed the evidence, saying that any risk of drugs' being destroyed was the result of the decision by the police to knock and announce themselves rather than to obtain a warrant.

Lawyers for Kentucky and the federal government told the justices on Wednesday that the lower court had erred. There had been no violation of the Fourth Amendment, which forbids unreasonable searches, they said, because the police had acted lawfully every step of the way.

Justice Elena Kagan expressed doubts about that approach.

A standard that looks only at the lawfulness of police behavior, Justice Kagan said, "is going to enable the police to penetrate the home, to search the home, without a warrant, without going to see a magistrate, in a very wide variety of cases."

All the police need say, she said, is that they smelled marijuana and then heard a noise. "Or," she continued, "we think there was some criminal activity going on for whatever reason and we heard noise."

"How do you prevent," Justice Kagan asked Joshua D. Farley, a Kentucky assistant attorney general, "your test from essentially eviscerating the warrant requirement in the context of the one place that the Fourth Amendment was most concerned about?"

Mr. Farley said that nothing the police had done in this case had violated the Fourth Amendment.

Justice Sotomayor was even more direct.

"Aren't we just doing away with 'Johnson'?" she asked.

Justice Ruth Bader Ginsburg asked why the police could not simply roam the hallways of apartment buildings, sniffing; knock whenever they smell marijuana; then break in if they hear something suspicious.

Mr. Farley said, "That would be perfectly fine."

Other justices appeared untroubled by the standard the government lawyers proposed.

"There are a lot of constraints on law enforcement," Justice Antonin Scalia said, "and the one thing that it has going for it is that criminals are stupid."

He said a sensible criminal would answer the door but decline to let the police enter without a warrant.

In a blog post, Orin S. Kerr, a law professor at George Washington University and an authority on the Fourth Amendment, said the case, Kentucky v. King, No. 09-1272, presented a tricky question based on murky facts.

But he said the police should not be allowed to take advantage of at least some of the circumstances their own conduct creates. Among those circumstances, he said, are the reactions of people who are made to believe that the police are about to conduct a forcible search of their homes.