THE INVESTIGATIONS of city hall are tightening the pressure on Mayor Barry. As someone against whom no charges have even been filed, he deserves a presumption of innocence. But a succession of allegations emerging in the public domain has set fair-minded citizens to wondering whether the integrity and effectiveness of city government have not been deeply impaired.

First it was a convicted drug dealer and friend of the mayor's named Karen Johnson refusing to testify to a grand jury about allegations of his and others' drug use and going to jail for contempt. Then came the revelation that she had been paid some thousands of dollars during and after her months in jail. One ensuing explanation, attributed to Karen Johnson, was that she had told prosecutors she received up to $25,000 for not testifying -- some of it from two prominent local contractors who are political allies of the mayor. Now these two, John Clyburn and Roy Littlejohn, are said to acknowledge giving payments but to insist that the money was offered out of sympathy and was unrelated to any refusal to testify. Meanwhile, nearly a dozen D.C. government officials have been convicted of crimes, and the FBI pursues an investigation of alleged corruption in city contracting.

One supposes that investigators are proceeding as investigators do, going to one person to find out about another person. The number and self-interest and high profile of the people who now have been touched by various inquiries are all considerations that tend to push elements of a private investigation into public view. It is necessary to keep in mind that the version of events appearing in public is, at the least, incomplete. Yet it is impossible to put out of mind that the mayor of Washington, who denies any suggestion of wrongdoing, remains month after month at the center of prosecutorial and public curiosity alike.

As an individual, Marion Barry deserves to be regarded as an innocent man. Yet as the city's leading public official he is called upon to provide not only good service but effective representation, and this has obviously become more difficult to do.

Currently, the public hears two strikingly different versions of a grave allegation -- paying off a witness in a criminal investigation. In one version a convicted drug dealer and friend of the mayor, naming names, says she was paid hush money. In a second version, the named, political friends of the mayor, are said to acknowledge making payments but to insist their purpose was entirely innocent. In these circumstances it is hardly enough for Marion Barry simply to stand on his uncontested rights as a citizen and to declare he has done nothing wrong. To restore full public confidence in his stewardship of the city, he is going to have to address these troubling inconsistencies head on.