Montgomery County police said yesterday they have renewed their search for a man known as "the laundry room rapist" whom they believe committed a series of rapes in the county three years ago. They said an incident last month apparently shows that the rapist is back in operation.

Police said they decided to reopen their investigation after a 31-year-old woman was raped Dec. 23 in the laundry room of her apartment building in Chevy Chase.

The woman was grabbed by the neck by an intruder as she returned to the basement to place clothes in a dryer, forced to undress and was raped, police said.

The woman's attacker then fled on foot.

"We can't say that this is the same individual, but there are similarities" in the case last month and those of three years ago, according to Cpl. Phillip B. Caswell, police spokesman.

Police believe that the "laundry room rapist" was responsible for a least 12 rapes that took place between 1974 and 1975 in the basement laundry areas of apartments in the Silver Spring and Bethesda areas.

Caswell said statements the rapist made to his victim last month made investigators think that he may be the same man they sought three years ago in one of the most intensive investigations in the police department's history.

One 1975 victim recalled of her attacker, "He had a mattress and everything all made up, waiting in the back. He talks like he's insane, asking me don't I love him. I guess he was imagining that I was involved with him."

Frequently, in the past laundry room rapes, the attacker would threaten his victims with a knife, but no weapons were used in the Chevy Chase incident, police said.

In the prior incidents, the attacker removed all the bulbs from the light fixtures in the washing machine area, leaving the hallway area lit, police said, and the women unaware the laundry room had been darkened.