They held a crate opening yesterday in the new East Building of the National Gallery of Art.

A crate opening is a media event. This one was for "The Splendor of Dresden: Five Centuries of Art Collecting," the 700-item loan show from East Germany, which, like the new building, will open on June 1.

Various VIPs - gallary directors, curators from Dresden, the East German ambassador, and cranrman Joseph Duffey, whose National Endowment for the Humanities helped insure the show - stepped into the light to have their pictures taken, and 100 members of the press elbowed one another go get a better view, as treasure-laden crates, which had bee opened once, were opened once again.

"Let's get to Christmas morning," said J. Carter Brown, the Gallery's director, as the first crate was unpacked.

A golden cup emerged. One could not see it clearly - a rope had been installed to keep viewers at a distance - but it obviously was a treasure. Unsmiling guards in uniform had been asked to stand beside it to protect it from the press.

The Gallery's new building, designed by I.M. Pei and given to the nation by Paul Mellon, his late sister and his family's foundations, is expensive too. Its cost has been anounced at an estiamted $95 million.

Yesterday, by accident, a few members of the press got a brief glimpse of its skylit central hall. They were supposed to take the back way in to the concourse galleries where the Dresden show will be installed, but some of them got lost.

For a moment they looked upward, past the marble-and-concrete balconies and bridges toward the gleaming, tetrahedral syklight. A hugh red-and-blue-and-black mobile by Alexander Calder moved slowly. The paving stones were triangles, the stairways seemed complex, the sunlight that poured down seemed very nice indeed.

The new special exhibition space is below ground level. The rooms, some round, some square, that the Gallery has built for the Dresden show do not reflect the architecture of the building overhead. They have temporary walls.

Already one wall has been dressed with arabesques of green and gold, and with small square mirrors. Its baroque design recreates the look of the Green Vaults of Dresden. The golden hunting cup, the first object uncrated, sits on a dying stag with a spaniel at its throat. Joachim Menzhausen, the Green Vaults' director, carefully unwrapped it and placed it on a wooden ledge jutting from the mirrored wall.

"No one will be allowed into the Dresden show without a pass," said Carter Brown. "The passes will be free. They will be distributed only at tents beside the building. They will tell you what time you may enter the exhibit. The idea is to minimize lines."

The second work uncrated was a jewel-encrusted pearwood statute of a smiling South American Indian carrying a large tray. His base is made of tortoise-shell; his breastplates are of gold.

"In 1581," director Menzhausen explained, "Emperor Rudolph II invited the Elector of Saxony to Prague, and there gave hime a present - a chunk of granite studded with huge emeralds that had just been discovered in the new colony of Colombia."

That rock, with its large gems, was then carefully unwrapped and placed on the smiling Indian's tray.

The first collections founded by the aristocrats of Saxony were full of natural wonders such as that chunk of stone. By the early 18th Century, they cared more for esthetics than they did for curiosities. They commissioned the two-foot statue, Menzhausen explained, "to make a piece of nature into a work of art."

Brown said the emeralds on the tray looked "like huge lollipops."

There are 20,000 pieces of rare porcelain in Dresden's state collections. One of these, a small blue vase, was removed from the next crate.

"I'm glad I'm not unpacking it. I'm a notorious butterfingers. My wife won't trust me with the dessert plates," said Brown.

An exotic porcelain monky, made in 1732 by the Meissen factories of Dresden for Augustus the Strong of Saxony, was then placed on view. Its base had been cracked in firing. "This crack," said Frau Menzhausen, who unwrapped it, "proves it is authentic."

Other costly objects - golden captured Turkish daggers, a crossbow set with lengths of stafhorn, old pistols, old swords and a gold parade helmet that looks like a fat eagle - also were displayed. The crate business was over, though. They already had been unpacked.

The last works shown were Old Master paintings by Rubens and Tintoretto.Cranach's splendid 1514 portrait of Duke Henry the Pious also was displayed.

Directors Brown and Menzhausen left the room together. "It went very well," said Brown.