A federal judge in Alexandria declared a mistrial yesterday after a jury was unable to reach a verdict in the trail of a U.S. park policeman accused of sexual bribery.

Officer Donald Phillip Percival was accused of having sexual relations with a woman in his police cruiser in exchange for refraining from charging her with drunken driving after he stopped her about 1 a.m. Nov. 30 on her way home from a Georgetown bar.

The defense attacked the woman's credibility, arguing that she was mentally unstable, and called the prosecution's case conjecture.

The jury deliberated for 14 hours over two days before informing U.S. District Judge Dortch Warriner late yesterday that it was hopelessly deadlocked.

"The case will not be droped," Assistant U.S. Attorney Theodore S. Greensberg said after the mistrial was declared. He said prosecutors plan to confer with Percival's lawyers today.

Percival and his wife, who sat through the entire trial, declined to comment as they left the courthouse. "We have a lot of factors to weigh," Dan Haynes, one of his lawyers told reporters. Percival was suspended from his job without pay after being charged in the case with accepting sexual favors in return for not issuing a violation notice, transporting a woman across a state line for immoral purposes and converting government property (his police car) to his own use in the incident.

In connection with the same Nov. 30 incident Percival has pending in Arlington a related state charge of forced sodomy for his alleged sexual relations with the woman driver.

The prosecution's case rested largely on the testimony of the 21-year-old Alexandria woman who said she was driving home Nov. 30 after an evening of drinking at a Georgetown night spot when Percival stopped her. The officer ordered her to a secluded area along the George Washington Memorial Parkway after telling her she was intoxicated, she testified.

Percival then had her get into his police cruiser and twice had sexual relations with her, the woman testified. She said she submitted without a struggle because she felt "it was in my best interest to do it," adding that Percival was armed.

Prosecutors sought to bolster the woman's testimony with a chain of circumstantial evidence. For example, butts found in Percival's patrol car after the incident were the same brand as cigarettes smoked by the woman, the government said.

Prosecutors also produced another woman who testified that in 1976 Percival let her go without charging her with drunken driving after he made sexual overtures to her during a late-night traffic stop.

Defense lawyer John F. Mardula said the prosecution's case was based on "conjecture and police guess work," adding that the charges against Percival were a "tragic mistake." Percival did not take the witness stand in his own defense.

The defense strongly attacked the woman's credibility, contending that she had history of mental instability marked by alcoholism, drug use and sexual promiscuity.