"Metaphor: New Projects by Contemporary Sculptors," opening at the Hirshhorn Museum today, is the most challenging exhibition of contemporary art to turn up in a Washington museum in more than a decade.

It is filled with heroically scaled, machinelike contraptions meant to arouse thoughts of society and creation, stage sets of high theatricality that warn of nuclear holocaust, and peaceful architectural environments that praise poetry and meditation. One of the most powerful pieces is the simplest: Lauren Ewing's prisonlike environment, which conjures -- in three simple black constructions -- the Orwellian issue of how power flows to those who control information.

This is public art that yearns to communicate ideas about the crucial themes of our day: war, power, control, creativity and social interaction. They begin by engulfing the viewer with wonderment and questioning: What are these things, anyhow? What do they do? What do they mean?

There are only six works, one to a room and each specially commissioned for this show. But the artists are six of the most inventive American avant-gardists now at work: Vito Acconci, Siah Armajani, Alice Aycock, Ewing, Robert Morris and Dennis Oppenheim.

They have been brought together here -- for the first time ever -- to make a point. As Hirshhorn curator Howard Fox says, "History may not conclude that each of these artists is as important as Picasso, but the mission upon which they have embarked -- given the minimal and formalist art that immediately preceded it -- is no less deliberate and courageous than the revolutions of the French Cubists or the Russian Futurists."

Ewing and most of the others use minimal abstract forms, but the results are a far cry from the mute, icy minimalism of the '60s. No single style prevails; what these works have in common is their use of visual metaphor, defined by the exhibition as "one reality seen in terms of another." Pioneer minimalist Robert Morris, for example, has done a complete turnabout and created an antinuclear polemic in the form of a black, shrouded stage-set with four helmeted black skeletons astride four warheads aimed straight at the viewer. Turn your back on this apocalyptic vision and you'll confront the consequences: four curved, funhouse-type mirrors in which your own image will be literally distorted and destroyed. The message is unmistakable -- too unmistakable, one might argue.

In sharp contrast, Iranian-born Siah Armajani from Minneapolis (the only non-New Yorker in the show) has built a more subtle work, an entire room designed for the peaceful purposes of thinking and meditating. Entitled "Hirshhorn Employees Lounge," it is filled with benches, reading desks, and a place to hang your hat -- literally and metaphorically. On the walls and baseboards are quotations from Walt Whitman -- "ready-made word pictures," Armajani calls them. This piece has meaning only when it is in use. The same is true of Vito Acconci's aluminum merry-go-round-like construction entitled "Fan City." When the protruding fins and roll-up banners are manipulated by the viewer, the piece rouses thoughts and feelings related to social interaction in a small city.

Alice Aycock -- the only artist who has had major visibility in Washington before -- has made the most elegant and purely sculptural piece in the show -- a wholly abstract construction, incorporating galvanized steel rotary fans and arcs and rectangles of glass that slice the space. It continues her pursuit of an elusive life force in a series titled "How to Catch a Manufactured Ghost." Dennis Oppenheim's giant contraption, "Anatomy of Celebration: A Launching Station," bears some visual resemblance to Aycock's piece, but is entirely different in intent. Its full impact, in fact, can only be imagined, because his plans to use the central imagery of live fireworks as metaphor for the thought process were aborted by Smithsonian security guards.

Revolutions, curator Fox admits, come in all sizes, and it is too early to tell what dimensions this one will ultimately take. But from first encounter with these highly philosophical works, it is clear that new expressive forms are being sought -- and found -- in the service of a new humanism. The pieces also share a breadth of expressive means: they communicate not only visually, but also intellectually and sensuously, often requiring the viewer's physical participation. Meanings are elusive, unspecific and mysterious, but the works speak. And they do it in a way that both extends and radically alters recent sculptural modes.

All of these works were constructed in situ, a process that took a great deal of vision and courage on the part of the Hirshhorn's administration. It also took the sparkling intelligence of Fox, who conceived and organized the show and its still-to-be-published catalog. Following, as it does, two other contemporary "Directions" shows, "Metaphor" suggests that we may have another welcome revolution under way: the emergence of a national museum of modern art in Washington that is both willing and able to intelligently engage the challenge of the new.

The exhibition will continue through Feb. 28, and an extensive catalog is in preparation. Tonight at 7:30 in the Hirshhorn auditorium, Fox will moderate a public discussion with the artists. Admission is free.