Barry Diller resigned today as chairman of Paramount Pictures Corp. to run 20th Century-Fox Film Corp.

The announcement followed by less than 24 hours the announcement that Alan Hirschfield, the chairman of Fox, is resigning to form an investment partnership with Marvin Davis, the Denver oil man who owns controlling interest in Fox.

The two moves -- combined with a reorganization of Paramount, which its owner, Gulf & Western Industries Inc., said it will announce Wednesday -- and several other recent top-level Hollywood shifts could dramatically alter the Hollywood power structure. Changes in powerful executive posts there can lead to new alliances between the studios and the actors, directors and others who mold the nation's entertainment.

The developments come on the heels of the ouster Friday of Ronald W. Miller, the chief executive of Walt Disney Productions. The leading candidate to succeed Miller, who had been with the Disney organization for 30 years, is reportedly Frank Wells, now vice president of Warner Bros.

Hirschfield was forced out as chairman of Columbia Pictures Industries Inc. six years ago in the fallout from embezzlement allegations against David Begelman, then a Columbia executive. The Begelman matter was the subject of a best selling book, "Indecent Exposure."

But Hirschfield's performance at the helm of Fox was lackluster, with the studio gaining little ground in either the film or television markets. The Fox feature-film operation now has its third chief in less than two years, and observers say the management instability has resulted in a strategy of picking up films made outside the studio, a characteristic of most of the Fox hits of recent years such as "Silkwood." Among Fox's recent failures is "Rhinestone," featuring Dolly Parton and Sylvester Stallone.

"Fox has been in a morass for some time," said analyst Alan Cole-Ford of Paul Kagan Associates. "Fox has been more of a picture picker than a picture maker. There has been no real management continuity and no consistent product flow. These changes are best of all for Fox."

Diller was part of a three-man triumverate with Michael Eisner, Paramount's president, and Frank G. Mancuso, president of Paramount's motion picture division, that won praise for spotting trends and successfully marketing films. Paramount widely is considered the most successful television and film production company of the 1980s.

Among the Paramount successes during Diller's tenure at the studio were "Terms of Endearment," "Trading Places" and "An Officer and a Gentleman."

In announcing Diller's departure, Gulf & Western said its chairman, Martin S. Davis, and senior officials met last week to plan the reorganization of the conglomerate's Entertainment and Communications Group.

Fox's Davis said in a statement that the Diller appointment represents the studio's "commitment to develop quality entertainment product in motion pictures, television and other media. We intend to support his efforts with whatever resources are necessary to make his role here a successful one."

No details about Diller's contract were released. At Paramount, where his contract expires Sept. 30, Diller was earning more than $1 million a year.