If consistency were consistently a virtue, there might be some defense for the CBS movie "Guyana Tragedy: The Story of Jim Jones." Certainly the film is of a piece -- totally, unforgivably and unconscionably gross. It need never have been made and should never have been made for television.

We may be resigned by now to large amounts of craven exploitation in the entertainment and publishing businesses. But "Guyana" -- tonight and tomorrow night at 9 on Channel 9 -- goes beyond reasonable limits of tolerance. It amounts to highway grave robbery.

Perhaps it will so sicken a seemingly densensitized national audience that there will be a productive reactionary backlash against similar acts of ghoulish opportunism in the future.

One usually, perhaps unfairly, associates ABC with descents this tawdry in the pursuit of ratings and money. But CBS, now neck-and-neck with ABC in the seasonal ratings race, has outdone all its rivals in stooping for lows with "Guyana."

In fact, CBS has with some regularity shown a compunction for luring viewers with gruesome titillation. This is the network that turned the Manson massacre into a multipart TV spectacular called "Helter-Skelter" in 1976. It won big ratings and convinced executives that there's nothing like a mass murderer to slay the competition.

Earlier this season, during crucial ratings sweeps, CBS hoped to make another killing with showings of "The Exorcist" and "Exorcits II: The Heretic," two films whose ghastliness was not much altered by TV editing. In fact, a Warner Bros. executive lied a couple of years ago when he said "Exorcist was too strong to be shown on TV in any form in the foreseeable future.

CBS had already bought it.

"The Exorcist" and its sequel did worse in the ratings than expected.But the morbid curiosity tune-in on the four-hour "Guyana" may prove a bonanza for sponsors buying commercial time. And it has been ironically suggested in print that the show could put CBS into the No. 1 position in the Nielsens and thus return it to the throne of "the Tiffany's of networks." t

Well, if so they can cower all the way to the bank. "Guyana" represents the lowest form of television.

Ernest Tidyman based his screenplay on "Guyana Massacre: The Eyewitness Account," a quickie paperback produced by reporter Charles A. Krause and other Washington Post staff members in 1978. The book was a factual if disorganized account of an earth-shaking atrocity. The movie which takes little from the book but much from other sources, is unblemished by social or informational value. It takes no point of view other than lurid astonishment. It makes no attempt to place the events in Guyana -- in which more than 900 people died and a U.S. congressman was murdered -- into any kind of fruitful perspective.

Producers dealing with true material this distasteful and incendiary have to justify bringing it all up again. NBC did that, to its credit, with "Holocaust," a program whose esthetic credentials may have been meager but that can safely be said to have stirred the conscience of the world, "Guyana" stirs only the stomach.

It is typical of the producers' and the network's attitude that the film opens with a cheap scam. We are in Guyana, a caption says, and we hear sirens and screams as people run in panic and a bird flies off from its perch. "Quick, drink the potion . . . They've come to kill us!" a voice cries out. A woman is shot and blood splatters on the front of her dress

Then the simulated voice of Jones says over a loudspeaker, "The alert is over; it was just a loyalty test . . . We must always be prepared to take the final step." Ha ha -- it was all a joke. He might as well have added, "Stick around, folks, for the real thing later in the show,"

Obviously if Tidyman (and director William A. Graham) had just told "The Story of Jim Jones" chronologically, viewers would have to wait four hours to see a bloodbath. So they cynically play a dirty little trick to hook 'em from Minute One.

Cynicism is the motif of this production. It was constructed so that there is some bit of violence, hysteria or sexual kinkery in almost every segment.Part one tonight ends with a burst of florid sensationalism not unlike the incestuous tease that provocatively concluded part one of the CBS movie "Flesh and Blood."

At the halfway point in "Guyana," Jones is in a room with a husband and wife, both of whom he has allegedly known sexually. The woman is repelled by this realization and Jones tells her, "I want you to see this" before embarking on a homosexual act (suggested by the onset of a kiss) with the man.

Perhaps Tidyman feels all the world shares his cynical Hollywood outlook, since at one point he has the late Rep. Leo J. Ryan almost licking his chops over the prospect of going to Guyana because the trip "could make headlines all over the country."

Decorating the ordeal with actors of such high caliber as LeVar Burton, Ned Beatty, Colleen Dewhurst, James Earl Jones (for about four minutes as Father Divine) turns "Guyana" into an all-star trash-out. Powers Boothe has a certain imposing magnetism as Jones, but the script is totally incoherent on the subject of his metamorphosis into a monster.

The mishmash of flashbacks includes scenes of Jones as a child in Lynn, Ind., where he chants "Sinners! Sinners! Sinners!" to other kids, preaches sermons to dogs, and reads the Bible in bed while his parents fight in the other room. Tidyman seems to be setting up a courtroom case for temporary insanity, but even this theme is dropped in deference to prurience.

In the Tidyman version -- the Tidy Bowl version, really -- Jones himself does not order the murder of Ryan and his party but seems vaguely aware it is taking place. Exactly who fired the bullet that killed Jones is also fudged. Since other liberties of dramatization have been taken, it would hardly have been unruly to make specific conjecture.

The story has not been given dramatic shape or tragic stature. It does not investigate human conditions. It uses headlines as an excuse for parading shocks and grislies across the screen. "Once he was a fine man, but something happened to him," a lapsed follower says of Jones at one point; it's an acurate summation of Tidyman's foggy notion of a thesis.

Last year in Ojai, Calif., a number of prestitious producers and writers got together in solemn conference because they feared that the docu-drama was in danger of extinction. As they watch the lovingly but pointlessly recreated horrors of Guyana on CBS, they can ask themselves whether this is the kind of useful, thoughtful,worthwhile entertainment they hoped to preserve.

CBS Entertainment President Robert A. Daly has promised a champagne party for co-workers to celebrate the network's high ratings this year. Perhaps if the numbers are good on "Guyana," he can raise a glass to Jim Jones at the same time -- for auld lang syne, and profitus maximus.