HOLLYWOOD COVETS ITS MYTHS, and it’s been a very long time since two films have engendered as much instant mythology as have “Jurassic Park” and “Last Action Hero.”

Consider “Jurassic”: If it lives up to its $ 200 million forecasts for the U.S. and obliterates “E.T. the Extra-Terrestrial” records overseas, the ’90s will be steeped in sequels, theme parks, dinosaur merchandising ploys and enough corporate synergies to fill the Harvard Business School library. Suddenly MCA becomes the hot studio once again and once-irritable Matsushita makes an obsequious bow to its cherished Citadel of Software, Universal City.

Sony, too, has come to look upon Hollywood as its savior, not a thorn in its corporate coffers, and the fate of “Last Action Hero” will play a major role in Sony’s planning for the ’90s.

Here is what’s on the line: two films that together cost roughly $ 135 million to produce with an additional $ 60 million being lavished on advertising , merchandising and various arcane tie-ins that seemingly involve every fast-food emporium in the continental United States. More than money’s at stake: also careers, corporate reputations and dynastic pride.

Given the big bucks involved, the nation’s critics will doubtless review the budgets as well as the films, despite the fact that the money was spent in sharply contrasting ways. Dinosaurs are the stars of “Jurassic,” which sent the below-the-line spiraling up to nearly $ 45 million.

“Last Action Hero,” by contrast, has one of the biggest above-the-line price tags of any film in memory, perhaps in excess of $ 30 million. This is due not only to the $ 15 million paid Arnold Schwarzenegger but also the considerable sums lavished on various producers and rewrite men.

ONCE THE NUMBERS SPIRAL OFF to stratospheric heights, of course, the projects become not so much movies as corporate product lines. One not-surprising result: While both films excel, indeed dazzle, in terms of special effects, critics already are pointing out that the characters somehow got lost. In “Jurassic,” writes Julie Salamon of the Wall Street Journal, “the dinosaurs feel real but the humans seem fake.”

As though anticipating this problem, “Last Action Hero” poses a surreal solution. “Hero” may be Hollywood’s first megapic in which the characters don’t even try to seem real. Schwarzenegger plays Schwarzenegger in much of “Hero,” only to flash in and out of a fictional character named Slater, and at any given time the audience has to figure out whether they’re watching a movie or a movie about a movie.

The two projects thus constitute a dizzying convergence of movie with theme park. Traditionally Hollywood filmmakers sought to synthesize reality, but the folks who made these two megapix are trying to invent a skewed form of virtual reality. This is an easier task in “Jurassic” since it’s easier to achieve verisimilitude with velociraptors. It may prove more difficult with “Hero,” which reaches for new heights of tongue-in-cheek jeopardy.

Since both projects are treading on fascinating new turf, it was inevitable that the Hollywood hot line would be churning with myth and rumor. One rumor involved the “Mysterious Pasadena Preview.” According to various alleged witnesses, “Last Action Hero” was shown to a test audience a week ago, producing very disappointing results. True? Not quite. In fact, a film was previewed in Pasadena, but it was “Rising Sun,” the upcoming Fox project, and this information never overtook the rumor.

THEN THERE WAS THE GEORGE LUCAS rumor on “Jurassic.” Since Steven Spielberg had immersed himself in “Schindler’s List” soon after completing “Jurassic,” his pal Lucas was really responsible for the editing, thus explaining the film’s “hard edge.” Had Spielberg been more involved, according to this theory, the dinosaurs would have been a kinder, gentler species, and some of the stunts would have been less horrific.

Not true, insist the experts. Following the close of principal photography on “Jurassic,” Spielberg had two months to organize his cut, and he continued to supervise post-production via an extraordinary satellite hookup with his location in Poland. Lucas functioned more as coordinator, but the style and the sporadic violence bear Spielberg’s stamp.

Finally, there’s the rumor about the “famous flip” whereby Columbia had decided to substitute its Clint Eastwood film, “In the Line of Fire,” for “Last Action Hero,” thus delaying the Schwarzenegger blockbuster until July 9 to permit time for tests and tweaks. The logic made sense: The Eastwood picture finished shooting almost two months before “Last Action Hero” and had acquired a very good buzz.

The supposed flip could have saved millions in post-production golden time, and Schwarzenegger wouldn’t even have had to hire a masseuse to minister to stressed-out technicians. The only trouble was Columbia never even considered it. From Peter Guber on down, there was a strong conviction that the Schwarzenegger epic should “open the summer,” irrespective of the cost.

Indeed, the only rumor about the two pictures that proved to be true involved the initial sale of “Jurassic Park.” Rather than go through the ritualistic auction, CAA set a formidable $ 2 million price on the Michael Crichton novel and circulated it to six studios and a handful of top filmmakers. Of the $ 2 million, $ 500,000 would go toward a Crichton first draft. Almost every studio said “yes,” and it was left to Crichton to anoint a filmmaker of choice. Not surprisingly, he selected Spielberg.

Last Friday, even as the “Jurassic Park” mushroom cloud hovered over 3,500 screens, a few well-guarded copies of Crichton’s latest novel were quietly circulated around town. This book, however, has no dinosaurs and no theme parks. Crichton, clearly, wants to venture on to new turf. The question remains where Hollywood will go after “Jurassic” and “Hero,” a question with vast financial as well as artistic ramifications.