ASHINGTON, April 5 — The Supreme Court agreed on Monday to decide whether the police need a special reason to subject a car they have stopped for a traffic violation to a drug-sniffing dog.

The case is an appeal by the State of Illinois from a ruling by the Illinois Supreme Court. That court found last year by a vote of 4 to 3 that use of a drug-sniffing dog, without any particular reason to suspect that the car contained drugs, had unconstitutionally broadened the scope of a routine traffic stop. In this case, the dog alerted the police to a large quantity of marijuana in the trunk.

The United States Supreme Court ruled in 1983 that exposure of luggage at an airport to a trained dog was not considered a search under the Fourth Amendment's prohibition of unreasonable searches. The canine sniff is in a class by itself, Justice Sandra Day O'Connor said in that case, explaining that the sniff was less intrusive and revealing than an ordinary search.

For that reason, Illinois is arguing that in the new case, the State Supreme Court was incorrect to find a Fourth Amendment violation in the addition of a trained dog to a legitimate traffic stop. The driver, Roy I. Caballes, was stopped by a state trooper for going 71 miles per hour in a 65 m.p.h. zone.

"A Fourth Amendment non-event (here, a canine sniff) remains a Fourth Amendment non-event even if conducted during a legitimate traffic stop," the state argued in its brief.

The Illinois Supreme Court did not maintain that a dog sniff, by itself, was a search. Rather, the court said, the use of the dog "impermissibly broadened the scope of the traffic stop in this case into a drug investigation because there were no specific and articulable facts to support the use of a canine sniff."

The trooper who stopped Mr. Caballes originally told him that he would get a warning. The trooper asked for consent to search the car, and Mr. Caballes refused, as was his right under the circumstances. While the trooper was writing the warning, an officer appeared with a drug-sniffing dog, which immediately alerted the officers to the trunk. The trooper searched the trunk, found the marijuana and arrested Mr. Caballes.

After the trial judge rejected his objection to the search, Mr. Caballes was convicted of drug trafficking and sentenced to 12 years in prison and a fine of $256,136, representing the street value of the drugs.

Four years ago, the Supreme Court invalidated a drug-detection roadblock program in Indianapolis in which the police stopped cars at checkpoints and subjected them to drug-sniffing dogs. Without addressing the use of the dogs, the court found that the Fourth Amendment did not permit roadblocks for the general purpose of crime control.

The new case is Illinois v. Caballes, No. 03-923.

These were among the court's other actions on Monday:

International Tax Case

The court agreed over the Justice Department's objection to hear an appeal by three men convicted of federal wire fraud for a scheme to smuggle liquor into Canada from Maryland without paying Canadian liquor duties and excise taxes. The question is the validity of a federal prosecution for the avoidance of foreign taxes.

Under a doctrine known as the revenue rule, which dates from English common law and is generally recognized, although not codified, by American courts, courts generally will not enforce foreign tax judgments. This doctrine has been invoked by a number of courts recently to dismiss suits by foreign governments to recover taxes they claim have been lost to cigarette smuggling originating in the United States.

The new case, Pasquantino v. United States, No. 03-725, is atypical in that it involves a federal criminal prosecution. The wire fraud statute makes it a crime to use interstate communications — in this case, telephone calls — to obtain "money or property" by fraud or false pretenses. The specific legal issue is whether Canada's potential tax claims amount to money or property under the statute. The United States Court of Appeals for the Fourth Circuit, in Richmond, Va., upheld the prosecution. Other federal appeals courts have disagreed in other cases.

The Justice Department, while generally supporting the revenue rule, maintained in its brief that the rule did not apply to this case, where the prosecution sought not to collect taxes but to "vindicate the United States' sovereign interest in preventing persons from using the wires in this country to commit fraud."

Wetlands Regulation

Without comment, the court turned down three cases challenging federal regulatory power over wetlands that are not directly connected to navigable waterways. Landowners, supported by the building industry, contested the government's interpretation of the Clean Water Act in light of a 2001 Supreme Court decision that rejected federal jurisdiction over isolated ponds visited by migratory birds.

According to the Environmental Protection Agency and the Army Corps of Engineers, that decision was a narrow one that did not remove federal jurisdiction over wetlands that are part of the drainage area or tributary systems of navigable waterways. The plaintiffs and their allies pressed for a broader interpretation of the 2001 ruling.

One, John A. Rapanos, a Michigan landowner who acted without a permit to fill wetlands that were 20 miles from a navigable river, was criminally convicted and now faces a 10-month prison sentence. His appeal was Rapanos v. United States, No. 03-929. The others were Deaton v. United States, No. 03-701, and Newdunn Associates v. United States Army Corps of Engineers, No. 03-637.