Cathy Evelyn Smith, the former backup singer scheduled to begin trial tomorrow for second-degree murder in the 1982 drug overdose death of John Belushi, will instead plead guilty to involuntary manslaughter, attorneys in the case announced yesterday.

The plea bargain ends four years of investigation and speculation into the death of the comedian, actor and writer who became a star on "Saturday Night Live" and went on to the movies "Animal House" and "The Blues Brothers."

Smith was the last person to see Belushi alive. The Los Angeles District Attorney's office argued in preliminary hearings that Smith, then 35, injected Belushi, 33, with the cocaine and heroin that killed him.

Her attorney, Howard Weitzman, who negotiated the current agreement, maintained that others may have held the needles and that Smith, a former heroin addict, was herself a victim. At the preliminary hearings he called her "nothing more than a lackey, a gofer, someone Mr. Belushi managed to manipulate."

The agreement also includes guilty pleas to three drug charges; nine other drug counts against Smith will be dismissed.

"This is the same plea offer that was made to her a year and a half ago," prosecution spokesman Al Albergate said.

Neither Smith nor Weitzman could be reached for comment late yesterday.

Albergate said the two sides did not reach an agreement on Smith's sentence, which will be decided by the judge.

The maximum term for the reduced charges is eight years and eight months in prison. Deputy District Attorney Elden Fox said he would ask for a sentence of no more than three years. The original charges carried a maximum term of 25 years to life in prison.

Smith had been questioned and released by the police the day Belushi was found dead, March 5, 1982, in a bungalow at the Chateau Marmont Hotel on the Sunset Strip. A Canadian native who sang backup for such recording artists as Gordon Lightfoot, Smith then went to Canada.

While there, she gave an interview to the National Enquirer in which she was quoted as saying, "I didn't mean to do it, but I was responsible for his death." The National Enquirer paid her $15,000 for the interview, and she later recanted the statement.

According to testimony at the preliminary hearing, Smith had been under the influence of alcohol and drugs when she spoke to the National Enquirer reporters. But Smith also told other reporters that she had injected Belushi with cocaine and heroin many times in the last five days of his life.

The interview spurred the D.A.'s office to reopen the case, and in 1983 Smith was indicted. She remained in Canada until January 1985, when she agreed to return to Los Angeles as part of the plea bargain. But by then Weitzman, who previously won acquittal for former carmaker John DeLorean on federal cocaine charges, had taken over her case and on his advice she soon rejected the plea.

Those negotiations dissolved because prosecutors refused to rule out a prison term for Smith. Speaking before a Superior Court judge, Weitzman said, "I can't in good conscience, based on conversations with Miss Smith . . . make a recommendation that she enter a plea of guilty to any homicide."

Fox would not speculate about why Smith changed her mind and accepted the plea, but he did say, "His Weitzman's position on the case hasn't changed. He still doesn't think she should serve a prison term. But I think there's a certain recognition of the likelihood of conviction on several counts which would expose her to a lengthy prison term."

Fox emphasized the importance of a tape of an interview introduced during the preliminary hearing in which Smith said in the last 24 hours of Belushi's life she gave him more than 20 injections of a mixture of heroin and cocaine known as "speedball."