In the annals of great American secrets, few have been as religiously kept as the answer to the question "Who Shot J. R.?"

That is supposed to be revealed to a waiting nation tomorrow night on CBS, and vast forces have been mobilized to make sure that nobody, but nobody, finds out ahead of time.

Even in Las Vegas, where there's supposed to be an "inside scoop" on just about everything, there is no authoritative word on Who Shot J. R. The betting is heavy and the odds have been changing. "This is the biggest thing that's ever happened," says Sonny Reizner, sporting book director for the Castaways resort in Las Vegas. "People are calling in from all over the world. J. r. is bigger than Ali here."

Current odds have J. R.'s wife, Sue Ellen, and her boyfriend, Dusty Farlow, as the favorites at 3-1. J. R.'s father is holding steady at 10-1, and even J. R. himself is listed at 20-1. Other possibilities include Miss Ellie at 8-1; brother Bobby and his wife, Pam, and Sue Ellen's sister, Kristin Shepard, all at 5-1. The long shot in the field is J. R.'s infant son, John Jr. at 101-1.

In London, where "Dallas" is watched at least as avidly as it is in the United States, betting will cease at 4 p.m. tomorrow, even though the Brits will not get to see the show until Saturday. The British bookies have deduced that overseas phone calls tomorrow night would throw a monkey wrench into the high rolling.

Only a chosen few are privy to the solution of a mystery that has plagued Americans since last spring, when Texas oilman J. R. Ewing, played by Larry Hagman, ended the season on the smash hit "Dallas" with two bullets in his chest.

"Obviously, our aim right now is to prevent anyone from finding out who shot J. R. We are trusting nobody but ourselves with the security procedures, and it appears to be working," says Lee Rich, whose Lorimar Productions makes the show for CBS.

Lorimar, which still has possession of the film, will not reveal how it is going to be transported to CBS. In fact, they're not even telling when the film will change hands, or who will make the drop.

"Let me just say this," waffles Tom Bishop of Lorimar. "We are not using conventional means. The usual procedure is simply to give the film to CBS in L.A. and have them feed it by closed-circuit to New York. That will not be done this time."

What will be done? "I can't answer that." End of conversation.

Obviously, some people know. A Lorimar official says that only eight or nine individuals in the whole wide world have seen the show as it will actually air on Friday. Among them, the Lorimar spokeman hinted, are Gene F. Jankowski, president of the CBS Broadcast Group, and one CBS staff producer who has to put the commercials in.

And William S. Paley, president of CBS Inc., who wants to know everything .

During filming of the episode, elaborate precautions were taken. Some scenes were filmed in several different ways in order to confuse the cast and crew. A closed set was the order of the day.

At CBS they're not saying who knows what, but a spokesman says that "the portion of the program where the killer is revealed will not be inserted until the very last possible moment. We don't want any mistakes."

Anything shown on a network has to pass through the standards and practices department for the censor's seal of approval. But this lead proves to be a dead end, too. Head censor Donn O'Brien, asked if he would be reviewing -- or has seen -- the episode himself, replied, "Absolutely not! I talk in my sleep."

O'Brien gave the assignment to a member of his staff on the West Coast, but won't reveal her name, either. "The poor girl has had to stop wearing her Dallas T-shirt because her neighbors were driving her crazy with questions," he says.

Meanwhile, the game of cat-and-mouse betwen the press and the network goes on. Earlier this week, an exasperated CBS-owned KNXT-TV in Los Angeles finally gave up trying to find out when the film was going to be delivered, a spokesman says, because reporters couldn't crack the security. The station finally settled for an interrogation of the guards at the Lorimar gates.

At this moment, the top-secret "Dallas" episode is probably out of the film lab, has been returned to Lorimar, and either recently was or soon will be delivered to the CBS Television Network.

Yes, somewhere a man in a raincoat may be standing on a shadowy street corner waiting for an unmarked armored car to approach him with a mysterious parcel in a plain brown wrapper that will be flown on a private jet, with an escort of F-14's, to New York City and then taken by limousine to CBS network headquarters where. . .

For all we know.