The 12-year-old girl, the first witness yesterday at the trial of a Silver Spring man accused of child sex abuse and drug charges, spoke softly and in one-word answers to most of the prosecutor's questions.

But she described in detail how the defendant, Paul C. Black, had prepared crack cocaine and demanded sex from the girl and her 13-year-old friend in exchange for drugs.

"You don't get this stuff for free," the girl said Black told her when she and her friend visited his Silver Spring apartment on Oct. 9, 1989.

The girl, who was then a sixth-grader, said Black then asked both girls to perform sexual acts with him.

The girl said she and her friend, both of whom lived in Prince George's County, spent about an hour in the basement of Black's house in the 8700 block of Flower Avenue.

But Black's defense attorney, Maureen Essex, attacked the victims' credibility in her opening statement yesterday to a Montgomery County Circuit Court jury.

Essex said both girls had lied to authorities about the incident. Essex said the girls' stories contradicted each other in several areas, such as which girl performed the alleged sexual acts and the type of drugs Black allegedly offered the girls.

On the witness stand late yesterday, the 13-year-old girl denied involvement in sexual acts with Black and said the girls only smoked marijuana.

The 13-year-old girl also disputed the younger girl's assertions that another man had been present when the sexual offenses allegedly occurred.

The mother of the 13-year-old girl reported the incident to police. Black is on trial before Judge Jerry H. Hyatt on charges of one count of third-degree sex offense and two counts each of second-degree sex offense, contributing to the delinquency of a minor, and distribution and possession of controlled substances.

Assistant State's Attorney Frank Molony, in opening statements yesterday, told the jury that the allegations were "horrible and disgusting. It's a hard thing to listen to because no one wants to hear this."

Molony said the younger girl's statements had been substantiated after a police raid in December at Black's house. Molony said authorities found cocaine and other drug paraphernalia in the basement and a bedroom.

If convicted of all the charges, Black could face up to 45 years in prison.