Two Montgomery County librarians were kidnaped at gunpoint yesterday by a man who entered their office, announced a robbery and took them to a secluded dam, where he raped one of the women, police said.

The three-hour episode, during which one of the women alerted a gas station attendant to their plight by silently mouthing the word "help," ended with a high-speed chase in Wheaton and the arrest of Robert Sherman Chenault of 1114 Tucker La., Silver Spring.

Chenault, 26, was charged with two counts of kidnaping and armed robbery and one count of rape. He was being held in lieu of bond in the Montgomery County Detention Center late yesterday.

According to police, the suspect entered the library about 9 a.m. and approached the counter where the women, ages 32 and 35, were working. The women told police the man displayed a handgun, announced a robbery and obtained an undetermined amount of money from the library cash drawer.

The women were bound and handcuffed and forced into a rented 1978 Dodge Colt, police said, driven to the Brighton Dam area, where the older woman was tied to a post and raped while the other woman remained in the vehicle.

About 11 a.m., the man and the two captives pulled into the Fairland Exxon Station to purchase gas. It was there the attendant noticed one of the women mouthing the word "help" and, after the car left, called police with a description of the car and the license number.