Judge Arthur M. Ahalt of Prince George's Circuit Court has ruled that a Washington Post reporter should appear before a special county grand jury investigating rapes and sexual assaults in the county jail, despite arguments by Post attorneys that she should not be required to appear.

Post attorneys, who received a copy of the opinion yesterday, said they disagree with the judge's interpretation of the law. They said the reporter, Loretta Tofani, may appeal the ruling.

The grand jury was convened after a series of articles by Tofani in September described in detail a dozen rapes and assaults in the County Detention Center in Upper Marlboro.

Tofani was issued a subpoena in October to appear before the grand jury. Since then, lawyers for the newspaper have argued without success that the subpoena should be set aside. The jury is scheduled to reconvene on Jan. 13.

Last week, Assistant State's Attorney Robert C. Bonsib told Ahalt he wanted Tofani's testimony to help obtain indictments from the grand jury. Washington Post attorneys countered that the reporter's own investigation into the county jail was continuing, and the secrecy of the grand jury proceedings would "chill" present and future sources, because they would not know what, if anything, the reporter had revealed.

"One of the reasons the rapists told me about their crimes, was that they knew I was a reporter, not a police officer," Tofani said yesterday. "My job is to expose the problem; it's up to others to correct it."

But Ahalt ruled Wednesday that Tofani had waived her privilege under the 1896 Maryland shield law by naming the sources in her stories. Rulings in other cases have limited that law "to protecting only the identity of sources of information," Ahalt wrote, and not the information itself.

Ahalt noted that the case before him presented "a confrontation between two important fundamental principles of our constitutional form of democracy: The right of the people to investigate criminal activity versus the right to publish."

But he cited a 1972 U.S. Supreme Court ruling that "the Constitution does not, as it never has, exempt the newsman from performing the citizen's normal duty of appearing and furnishing information relevant to the grand jury's task."