In a pair of decisions that affect the privacy rights of everyone from homeowners to Fortune 500 companies, the Supreme Court said yesterday that police and other government investigators may conduct aerial surveillance and take detailed photographs from the air without obtaining a warrant.

In one case, the court upheld the Environmental Protection Agency's power to fly over and photograph a Dow Chemical Co. plant after the company refused the agency's request for an on-site inspection. In the other, the court said that Santa Clara, Calif., police did not violate the constitutional rights of a marijuana-grower when, unable to see into his fenced-in back yard, they flew over it at a low level to gather information for a search warrant.

The court, in two 5-to-4 opinions by Chief Justice Warren E. Burger, said that such techniques do not infringe on the Fourth Amendment's protection against unreasonable searches, despite the fact that government agents would have needed a warrant in order to see the same thing by entering the premises.

The majority emphasized in both cases that the aerial surveillance gave the government an advantage no greater than that of ordinary citizens flying over the area.

"In an age where private and commercial flight in the public airways is routine, it is unreasonable for the grower to expect that his marijuana plants were constitutionally protected from being observed with the naked eye from an altitude of 1,000 feet," Burger wrote in the drug case, California v. Ciraolo. "The Fourth Amendment simply does not require the police . . . to obtain a warrant in order to observe what is visible to the naked eye."

Four dissenting justices, however, warned that the court was letting the government eviscerate constitutional protections against unreasonable searches through the use of modern technology.

"Aerial surveillance is nearly as intrusive on family privacy as physical trespass," Justice Lewis F. Powell Jr. wrote in his dissent in the marijuana case. "It would appear that, after today, families can expect to be free of official surveillance only when they retreat behind the walls of their homes."

"Rapidly advancing technology now permits police to conduct surveillance in the home itself," Powell wrote, "an area where privacy interests are most cherished in our society, without any physical trespass . . . . "

Powell also wrote a dissent in Dow Chemical Co. v. U.S., which, he said, puts privacy rights "seriously at risk as technological advances become generally disseminated."

He was joined in both cases by Justices William J. Brennan Jr., Thurgood Marshall and Harry A. Blackmun.

Legal experts said the decisions extend a line of cases in which the court has taken a restrictive view of what constitutes an unreasonable search, particularly a 1984 case holding that police do not need a warrant in order to search a field posted with "no trespassing" signs.

"I thought they were going to make a distinction between somebody growing 500 acres worth of marijuana and what's going on in the back yard of somebody's home," said Charles H. Whitebread, a criminal procedure expert at the University of Southern California.

Taken together, Whitebread said, the decisions "say to law enforcement, if they use airplanes they can pretty well do what they want."

"You don't generally start out the day saying, 'Gee, what's in the back yard that I have to worry about somebody seeing,' " said University of Illinois College of Law Professor Wayne LaFave, the author of a major treatise on criminal procedure. "I guess now we have to . . . expect somebody's hanging up there in the sky, too."

In the Dow case, the EPA was investigating allegations of air pollution at Dow's 2,000-acre chemical manufacturing complex in Midland, Mich. The EPA had conducted one on-site visit when Dow refused its request to conduct a second inspection and take photographs of the plant, where Dow maintained elaborate security precautions.

Instead of seeking an administrative search warrant, the EPA hired a commercial aerial photographer to take pictures of the facility from 12,000, 3,000 and 1,200 feet. Magnification of the photographs would permit identification of objects as small as wires half an inch in diameter.

In upholding a decision by the 6th U.S. Circuit Court of Appeals approving the EPA's action, Burger said that "the photographs here are not so revealing of intimate details as to raise constitutional concerns . . . . The mere fact that human vision is enhanced somewhat, at least to the degree here, does not give rise to constitutional problems."

In the marijuana case, which reversed the California Court of Appeals, the Santa Clara police received an anonymous tip that a man was growing marijuana in his back yard, but were unable to see into the yard because it was completely surrounded by two fences, one 6 feet high and the other 10 feet high.

Police saw the marijuana when they flew over the yard, and 73 plants were seized the following day on the basis of a search warrant.

The marijuana-grower's "expectation that his garden was protected from such observation is unreasonable and is not an expectation that society is prepared to honor," Burger wrote in overturning the lower court's ruling that the warrantless observation violated the Fourth Amendment.