Contemporary art has changed dramatically since the Hirshhorn Museum opened on the Mall 10 years ago.

So has the museum. Once a nervous newcomer on a block that proved notoriously hard to crack for Joseph Hirshhorn -- the museum's immigrant founder -- the Hirshhorn is now the firmly established, heavily attended national museum of modern art. It has also proved to be nearly as tongue-tied on the subject of really new art as all the rest of Washington's museums.

Or is it? Despite its conservative image, the Hirshhorn has chosen to celebrate its 10th anniversary, on Oct. 4, by opening the riskiest contemporary show of its life: a pioneering attempt to make some sense of the divergent trends of the past decade.

But "Content: A Contemporar Focus, 1974-1984" is not just another survey. Instead, it takes a point of view and presents a thesis: that during the museum's short lifetime, there's been a noticeable shift in attitude among progressive artists, and that the shift has been away from the cold issues of form and style (as in Pop, Op, Minimal, Conceptual) toward an overriding concern with meaning. The Hirshhorn calls it content.

After the emotional vacuum of the 1960s, artists once again felt the need to communicate about matters of life, love, nature, metaphysics and art itself. They also again confronted social issues -- political oppression, nuclear war, the environment. With their minds reopened, a whole new world of possibilities came flooding in. Where else could they go after modern art had reduced itself to metal squares lined up on the floor?

It turned out there were endless ways to go, and the more adventurous artists pursued them all. With old visual traditions shattered, they set out to evolve a new visual language based chiefly on new combinations of existing media: painting with sculpture, sculpture with photography, photography with words, words with video.

They also used minimal images, primal signs and symbols, their meanings -- not always explicit -- communicated by evocation, juxtaposition, metaphor or sometimes by placing viewers in an environment. For instance: Rob re-created for the Hirshhorn show, is designed to rouse real fear in those who walk through it. Terry Allen, at the other end of the emotional spectrum, warmly explores the subject of love-as-wrestling-match through a videotape set in a shamelessly romantic setting bathed in sensuously colored light.

Antiwar statements come in many forms, including Chris Burden's advancing army of 50,000 nickels, entitled "The Reason for the Neutron Bomb." Each nickel, mounted with a matchstick cannon, is meant to represent a Russian tank lined up aong the border between Eastern and Western Europe.

The themes are old, but the modes of expression are new, a fact vividly illustrated in the 1970s version of landscape art. Rather than making grandiose, nature-worshipping paintings (such as those of 19th-century Americans like Thomas Church), British artist Richard Long, in his characteristic "Bluestone Circle," carefully arranges on the floor real pieces of stone picked up on his long walks across England. By these simple means, he manages to evoke a real landscape. Washington artist Jim Sanborn, in a similar way, conjures the magnetic forces of the earth by using the real thing: a series of suspended lodestones and compasses.

These new forms of visual shorthand depend heavily on the psychological awareness of the viewer and a willingness to submit to messages conveyed by universal primal symbols. This is art made to engage the mind, the emotions and the body itself -- not to decorate.

It is also historical work representing the years 1974 to 1984, not art created just for this show. Many of the 126 artists were already prominent in the late 1960s and are now Hirshhorn, from page 10 middle-aged or older. Some -- such as Philip Guston -- are dead. The show is as international and varied as recent art has been but is united by what curator Howard Fox calls "the will to meaning."

Fox, Phyllis Rosen and Miranda McClintic, the three Hirshhorn curators who organized the show, had each organized one of the Hirshhorn's three earlier "Directions" shows, which periodically examined minitrends. Here, they have attempted to make a coherent statement about what ties those and other trends together.

Ten years ago, Hirshhorn Museum director Abram Lerner, who retires this fall, said: "We are not here to show Washington what's new. That is the nction of the Corcoran and the local commercial galleries. This is a national museum of modern art. Our constituency is national. We will bring avant- garde art to Washington only if we consider it to be the best contemporary art around."

The question now is: Has the Hirshhorn been showing any of its constituencies "the best contemporary art around" with any regularity?

If we accept this anniversary show as a summary of the most important changes in art over the past decade, the Hirshhorn must also admit to some shortsightedness. True, a fraction of these artists has been shown there previously, but probably only 10 percent. And only one or two of them, according to the curators, are represented by recent work in the museum's permanent collection. Every object in the anniversary show had to be borrowed from other institutions and individuals.

But the Hirshhorn showed considerable courage in undertaking this project, not only because of its scope and risk, but because it also hereby admits, by clear implication, what the museum has failed to do. Come to think of it, that's pretty mature behavior for a 10-year-old.

The show will continue through Jan. 6, 1985.