John Rideout, a 21-year-old Army veteran and short-order cook, was found not guilty this afternoon of raping his 23-year-old wife, Greta, after a violent argument in their northeast Salem home.

A jury of eight women and four men deliberated 21/2 hours before filing back into the big, wood-paneled courtroom that for the last week has been crowded with reporters, women's organization members and spectators who sometimes spent the recesses arguing about the difficult questions raised by this apparently unprecedented case.

The jury's unanimous verdict was met with a burst of applause in the courtroom. Greta Rideout was not in the room at the time. John Rideout-who maintained throughout the trial that his wife voluntarily had sex with him after a heated quarrel on Oct. 10-got up slowly, looking too stunned to smile, and shook his attorney's hand.

Rideout is believed to have been the first man in the nation charged with raping his wife while they were living together. (A 41-year-old Oregon man was convicted last June of raping his wife in Portland, but the couple was legally separated and living apart.) The Rideout trial, which began Dec. 19 and attracted such widespread attention that attorneys on both sides reported receiving letters and financial, contributions from across the country, was the first application of a new and previously untested provision of Oregon law.

Rideout had been charged with commiting sexual intercourse by forcible compulsion, which under Oregon law is first-degree rape. The state defined 'forcible compulsion" as "physical force. . . or a threat express or implied."

The Oregon criminal code, which specifies a maximum sentence of 20 years for first-degree rape included until 1977 a provision that "female" meant a woman not married to, or legally separated from, the rapist. In early 1977, that provision was dropped, making it possible for a wife to bring a rape charge against her husband even is they are still living together.

Iowa and Delaware have similar provisions-although the law apparetly has not been tested in those states-and such a law is scheduled to take effect in New Jersey on Monday.

Although Gary Gortmaker, the Salem district attorney who conducted the prosecution, said while the jury was out that he would consider a not guilty verdict an indication that the jury disapproved of the law, the Rideout acquittal does not legally change the Oregon statute.

Asked after the verdict whether he thought other married women would be discouraged from pressing rape charges, Gortmaker said, "I'm ready to prosecute any case the grand jury returns an indictment on. . . I don't think it will have that effect at all."

Rideout's attorneys, Charles Burt and Philip Kelley, argued that the Oregon law invaded marital privacy, was unconstitutional because it specified that only men could commit rape, and attempted to "impair contractual obligations" implicit in the marriage cotract.

Greta Rideout, 23, testified that early in the afternoon on Oct. 10 she and her husband began arguing about whether she would have sex with him. "'You are my wife and you should do-' somewhere along that line-'you should do what -I want,'" she said her husband told her.

She ran out of the apartment, she testified, but Rideout dragged her back in. She said he threw her on the floor, grabbed her around the throat, asked her if she would "cooperate," and hit her in the face when she said no. She submitted, she testified, because "I thought I would have a broken jaw if I was hit anymore."

John Rideout acknowlegded from the beginning that the couple had fought, that he had hit her, and that they had had sex after. But "she hit me first," he testified. He said she kneed him in the groin, and that when he hit her "I stopped myself because I realized that I was really agitated. . . I said 'Greta, I'm really sorry, I love you, I didn't mean to do it.'"

Several jurors interviewed after the trial said that the law itself had not been discussed during deliberations and that they voted for acquittal because they simply could not establish beyond a reasonable doubt that Greta Rideout's version of the story was correct.

"It was so hard to tell who was telling the truth," said juror Pauline Speerstra, 62. "A lot of (the jurors) felt there would be another case in the future and it would be a better case."

John Rideout was asked when it was over how he felt, and he stood in front of the television cameras in the courthouse hallway, speaking slowly, looking scared. He said he was pleased, of course. "I don't believe this has happened to me, to start with," Rideout said. "You have a lot of mixed-up confused ideas in your head. I'm only 21 years old and I have a long way to go."