Montogomery County police said yesterday that Herbert C. Cahalen was suspended for the police department last week after he altered urine samples of three other officers being tested for drug use by adding his own urine to the samples. An article in The Post on Sunday said the urine samples were altered by a different means.

The Montgomery County Police Department last week suspended a member of the special narcotics unit after he planted cocaine in urine samples of three fellow officers as a practical joke.

During a urine test that the department conducts at least once each month, cocaine was found in the samples of four officers, two of whom are in supervisory positions. But only one of the officers, Herbert C. Cahalen, had used the drug legitimately as part of his undercover dealings with drug pushers.

Although a police department spokeswoman, Nancy Moses, would not confirm the identity of the prankster, Cahalen admitted last week that he had spiked the samples for fun. Moses said that an officer had recently used cocaine as part of his undercover operations and expected his sample to come up positive. As a joke, he also put cocaine in the samples of three other officers who were not authorized to use the drug, she said.

Moses said it is uncommon for officers to use drugs, although they occasionally use them on undercover assignments. She said an officer who uses drugs during undercover work must report such activity to his superiors immediately afterward so that he can be protected if a urine sample is taken and returns positive.

Cahalen was suspended for three days without pay and then requested a transfer from the special investigations unit.

Moses said department officials are convinced that the three other officers had not been using cocaine and that the whole incident was the result of a prank.

"The reaction to the tests was, 'How can this be?' " Moses said. "We consider it a serious matter . . . to tamper with another officer's integrity."

The spot urine checks are made to protect officers from persons who might want to press false charges against them, and also to maintain integrity in the department.

Cahalen has been on the Montgomery County police force for seven years.