An Alexandria judge ruled yesterday that prosecutors may use a Northern Virginia doctor's statement that he had prescribed more than 2,500 doses of narcotics for a woman who died of a heroin overdose.

Alexandria Circuit Court Judge Donald H. Kent said that Dr. Joel L. Koslow, charged with unlawfully prescribing massive quantities of controlled substances for the woman, was not under arrest or coerced when he gave the statement. The judge rejected the physican's claim that investigators should have warned him of his constitutional rights against self-incrimination before he spoke to them.

Koslow, 46, indicted Dec. 3, acknowledged in the police interview Nov. 9 that he had been having a three-year affair with Nancy Moffitt, 33, of Fairfax County and couldn't refuse her requests for drugs, investigators have said.

The doctor has not been charged in Moffitt's death. An Arlington man, Donald J. Munley, 33, of 860 S. Greenbriar St., was charged with manslaughter shortly after Moffitt's Aug. 9 death. Munley's trial is scheduled for March 11 in Fairfax County Circuit Court, and Kent agreed yesterday to postpone Koslow's trial until sometime after that.

Koslow told Kent that when "about 11 people" came to his 4921 Seminary Rd. office with search warrants on Nov. 9, he believed he was under arrest and had no choice but to answer their questions.

William A. Hurst, an investigator for the Virginia Department of Health Regulatory Boards, testified: "I told him he did not have to talk to me, but it would be helpful."

Assistant Commonwealth's Attorney Randolph Sengel said Koslow's statement was an "important" part of the prosecution's case.

Koslow told Hurst and two other investigators that he knew Moffitt was addicted to drugs, but he could not refuse her requests for them, according to a report filed in court. Koslow denied that he had supplied narcotics in return for sex and said their relationship was based on love, the court papers said.

A specialist in internal medicine, Koslow continues to see patients, but has been barred by the State Board of Medicine from prescribing controlled drugs. The board has said it will decide later whether to revoke his license.