Mississippi Republican Rep. Jon C. Hinson pleaded not guilty yesterday to a charge of attempted sodomy and entered Sibley Hospital in Northwest Washington for what aides called "counseling and treatment" as Mississippi political leaders called for his immediate resignation.

Hinson, appearing in D.C. Superior Court in a necktie and tweed jacket, stood expressionless before Judge William S. Thompson while his attorney formally entered the not-guilty plea. Hinson was arrested Wednesday and charged with committing sodomy with a Library of Congress clerk in a public restroom on the sixth floor of the Longworth House Office Building.

Prosecutors reduced the sodomy charge -- a felony -- to the misdemeanor level of attempted sodomy yesterday. Officials in the U.S. attorney's office said such charges are routinely reduced to misdemeanors when consenting adults are involved. A trial date was set for May 4.

Hinson's congressional office later released a written statement from his administrative assistant, Marshall Hanbury, saying Hinson entered the hospital because of "dissociative reaction" brought on by the "intense emotional and physical exertion" of his first two years in Congress and a gruelling reelection campaign. The brief statement did not mention the charge against the congressman.

An official of the National Institute of Mental Health said "dissociative reaction" is a rare disorder covering a range of mental problems including multiple personalities and temporary amnesia.

Reaction to the Longworth Building incident in Hinson's 4th Congressional District, a conservative Bible Belt section of southwest Mississippi, was swift and unforgiving as former political backers and two daily newspapers called for his resignation.

"After the incident, I think he is sick and I think all he can do is resign," said W. D. Billy Mounger, a Jackson, Miss., oil dealer, who has been one of Hinson's primary financial backers.

Both the Jackson Daily News and the McComb Enterprise Journal called for his resignation in editorials yesterday.

"He has now pushed his own self-destruct button, he must resign," said the Daily News.

Hinson, a staunch conservative, was narrowly reelected last year for a second term after he called a surprise news conference to acknowledge that he was one of four men who survived a fatal fire in 1977 that killed nine persons at the Cinema Follies, a homosexual club and theater in Southeast Washington.

Hinson also acknowledged at the press conference that he had been accused of "an obscene act" on the grounds of the Iwo Jima Memorial, an after-dark gathering place here for some homosexuals. Both incidents occurred before his 1978 election. He told the press conference both he and his wife, Cynthia, hoped to "put these incidents behind us -- finally."

Mississippi State GOP Chairman Mike Retzer of Greenville said yesterday, "I think the party was fooled" and called for Hinson's resignation.

On Capitol Hill, meanwhile, House Republican leader Robert Michel of Illinois and two of Hinson's Mississippi Republican colleagues, Sen. Thad Cochran and Rep. Trent Lott said they believe Hinson ought to resign. "It would probably be in the best interest of all concerned if Mr. Hinson resigned his seat and concentrated on the personal problems he faces," Michel said.

Hinson's effort to attribute his problems to emotional and physical stress brought criticism from local and national gay leaders in Washington, who said he should either acknowledge being homosexual openly or deny it.

"Dissociative reaction is absolute meaningless psychiatric gobbledegook [in this case], it's a refusal to face up to oneself as a homosexual," said Franklin E. Kameny, a longtime gay rights advocate. "If Hinson's therapists are any good, they will attempt to bring him around to constructive terms with his homosexuality which is not going to go away."

Steven Endean, lobbyist for the Gay Rights National Lobby, said the arrest "certainly isn't helpful in our effort . . . to insure basic civil rights for all gay persons. Public sex in general is not something this organization approves of." He said, "The thing that bothers me is that 99 out of 100 gay people would not be in that situation, but those 99 are going to be judged based on one."

Hinson is the second congressman within six months to be arrested on a charge involving homosexual conduct. Last October, Republican Rep. Robert E. Bauman of Maryland was charged in the District with soliciting a 16-year-old male for sexual purposes. Bauman, a powerful conservative in the House, later agreed to enter a first-offender treatment program. He acknowledged having homosexual tendencies and attributed his problems to alcoholism. He was defeated for reelection in November.

A source close to the Capitol Hill investigation said the restroom was on the sixth floor of the Longworth Building in an isolated area away from most congressional offices. The restroom actually is two rooms, with a 12x12-foot vestibule separating the restroom facility from the hallway.

Capitol police said they placed the restroom under surveillance after receiving complaints that some homosexuals had been using it for sexual liaisons. It is illegal in the District for anyone, heterosexual or homosexual, to commit sodomy.

Capitol Police Deputy Chief Gilbert Abernathy said there was "physical observation" of two incidents Wednesday -- one involving Hinson and the library clerk, the other involving two other persons who also were arrested.