Jack Ronald Jones, one of two men convicted of the 1982 slaying of Stephanie Ann Roper, also pleaded guilty yesterday to raping her. Jones' plea in Prince George's County Circuit Court ends a three-year drama that spawned a victim's rights movement in Maryland.

Jones' plea came in the middle of his trial on charges of rape, conspiracy and illegal weapons use in connection with Roper's death. In exchange for the plea to the first-degree rape charge, Assistant State's Attorney Michael Whalen agreed to drop all additional charges. Circuit Court Judge Jacob S. Levin will sentence Jones on May 20.

Roper's parents, Vincent and Roberta Roper of Croom, said afterward that they are glad their ordeal is over.

Jones and Jerry Lee Beatty, both of Waldorf, were convicted two years ago of kidnaping, raping and shooting Roper, then setting her body on fire on April 2, 1982. The men had offered a ride to the 22-year-old Frostburg State art student after she had an accident on a dark, country road in Prince George's County. She was raped in Prince George's, and again in St. Mary's County, where her body was later found.

The two young men originally were charged only with the crimes in St. Mary's. Beatty pleaded guilty and Jones was convicted by a jury, but they escaped the death penalty when judges sentenced them to concurrent life sentences, making them eligible for parole after 12 years.

The sentences outraged friends of the Roper family, who formed the Stephanie Roper Committee to lobby for tougher sentencing procedures and victims' rights legislation.

Prince George's State's Attorney Arthur A. Marshall, who has closely aligned himself with the Roper Committee, then moved to try the men in Prince George's in an effort to lengthen their prison sentences. He won an indictment against each of them for the rapes that occurred in Prince George's County, although defense lawyers unsuccessfully argued before the Supreme Court that such a trial would be double jeopardy. It was that charge that Jones pleaded guilty to yesterday.

Beatty has also pleaded guilty to raping Roper in Prince George's.

Whalen, the prosecutor, said he would accept a sentence of life with all but 20 years suspended, which would run consecutively to the two life sentences plus 20 years Jones is already serving. Jones also agreed not to seek further appeals.

Beatty is also expected to receive another life sentence with all but 20 years suspended. Such sentences are likely to make both men eligible for parole consideration in about 24 years, court officials said.

Vincent and Roberta Roper said they thought the no-appeal issue was most important. "We can go home today and say we don't ever have to see Mr. Jones or Mr. Beatty in court again," Roberta Roper said.

With the Ropers at the helm, the Roper Committee has successfully lobbied the Maryland legislature to pass bills that: increased the amount of time a criminal sentenced to life must serve, from 11 to 19 years, before being eligible for parole; removed voluntary drug or alcohol use from the list of mitigating circumstances a jury considers in death penalty proceedings; and require that the victim's family be notified when an offender's parole hearing is held.

Vincent Roper said yesterday that he is satisfied that Jones and Beatty "had been held accountable for the things they did."