Baltimore Mayor William Donald Schaefer, always on the prowl for good ideas, visited the new New England Aquarium in Boston some years back and decided on the spot he wanted one of those for his town. As seems to be his habit, the mayor got his way and then some.

The new National Aquarium in Baltimore has been playing to general huzzahs and big crowds since it opened last month. The excitement is understandable. The praise is indiscriminate.

Schaefer bought the New England idea whole, from the waterfront location to the crisp concrete-cum-steel-and-glass exterior to the led-by-the-nose educational philosophy embodied in the exhibits. He also chose the same architect, Peter Chermayeff, chief architect of the Cambridge Seven Associates firm, to design his "fish tank," as now-embarrassed opponents used to call it.

Chermayeff was tremendously pleased with the opportunity to do again what he did before but on a larger scale and in a spectacular location. It's an architect's dream that rarely comes true. His excitement with the second chance is evident in the results: From the outside the building is a soaring, spirit-lifting gift to the city of Baltimore, a galvanic response to a site that in itself is a mixture of marvels both man-made and heaven-sent.

Unfortunately the inside story is different. Once viewers have climbed the staircase, bought their tickets and passed the threshold, they are locked into a rigidly programmed pedagogic path, a busy bombardment of sights and sounds resembling an escape-proof course in Ecology I. The Baltimore aquarium is a much better building than it is a museum.

Its signal successes are formal and symbolic. The site is a dream, a pier nestled in the water with a clear view of a wonderfully varied urban panorama -- from the gentle green slope of Federal Hill across the harbor to the crowded comings and goings of the Harborplace shops to the heterogeneous skyscrapers of the new downtown to a gritty ensemble of old (and mostly vacant) industrial buildings.

The pier is the visual key to the circle of Baltimore's revivified Inner Harbor, and Chermayeff's building locks everything into place. With the example of Jorn Utzon's Sidney Opera House clearly in mind -- that poetic succession of huge concrete roofs looking so like a parade of weightless sails in the Australian air -- Chermayeff had his building make a dramatic gesture that is at once playful, strong and apt.

The idea of including a tropical rain forest in the progression of exhibits was built into the plan from the very beginning, the architect recalls, and he seized upon it: thus the towering steel-and-glass pyramid that is the apex of his design. From there he was but a step away from conceiving its tag-along sibling, the smaller pyramid that serves as a transparent canopy for the raised entrance platform.

These light, clean-lined forms make a marvelous pair. Diagonals atop cubes, they lighten the concrete load of the building and they open it as it should be opened to the elements. The smaller one is like a dinghy to the massive sail and prow of its large partner. The whole concoction of contraries -- open-closed, glass-concrete, diagonal-perpendicular, active-passive -- has a powerful, nautical flair that is perfectly apposite to the building's contents and its setting.

Most of the drama is concentrated on the western facade, which makes the design a bit lopsided, but, seen from the west, it is a tremendous Baltimore view, the sharp silhouette of the aquarium playing against the massive presence of an old power plant with four giant black smokestacks hovering in the background. Credit Chermayeff for knowing that with the power plant as a foil he could hardly make his building too strong. (The power plant is now owned by the city, which, under no circumstances should let it disappear. Happily, Baltimore planners know this.)

Truth-to-materials purists -- if the breed is not extinct -- may cringe at the way Chermayeff blithely decorates the gray-blue concrete surfaces with thick layers of paint, but few would deny he does it well. The method in his madness is not so much to break up the mass by playing with formal elements as it is to signal the purpose of the building. Without being overly specific, he wants it to say that something festive is going on here, and it does so.

What the building delivers inside its distinguished shell, however, is often a noisy multimedia blitz of entertainment and education. The lesson itself is unexceptionable, and if a viewer leaves the aquarium with an enhanced awareness of the delicacy of the ecological balance, well, that's fine. But viewers are likely to leave with headaches, too, and not just because the building does not comfortably accommodate the large crowds it has been attracting. Waiting in line in front of a museum display is never pleasant, but in most museums you can shift attention elsewhere.

In Baltimore, to step out of the linear sequence of displays is to miss the point, and the architect, with his system of uni-directional moving ramps, does not allow you to forget this. Once you have left a given area in the Baltimore aquarium, it is almost impossible to retrace your steps, and as a result much potential pleasure is thrown away. This architectural determinism seems out of sync with both the function of the aquarium and the casual mood of the harbor setting.

Luckily, the Baltimore aquarium is divided into two parts, and the second provides a welcome antidote to the first. In the first area, Chermayeff overindulges a penchant for busy-ness in a hollowed-out space with narrow ramps zapping this way and that. He never allows a place to be without an image, a word or a sound. The architecture here so perfectly expresses an excessive educational zeal that it is a pleasure at last to step into the high-flying rain-forest tower, from which, in addition to everything else, there is an exhilarating view of the harbor.

Then comes the relief of the "ring tank," a dim, cavernous space in which viewers, free to go their own ways at their own pace (but not without the accompaniment of a sort of marine Muzak), are surrounded on all sides by beautiful fish, first in the wonderful coral reef exhibit and then, further down, in the "open ocean tank" with its unforgettable sharks.

A little more of this kind of restraint would go a long way to relieve the tedious cacophony of the long string of initial exhibits. Chermayeff's clear belief in "a linear educational philosophy" also contains some good old show-biz logic: give 'em the sharks and dolphins, and they'll swallow the rest of the message.

But if a visitor leaves the building with mixed feelings, the splendid setting reasserts itself outside. If he looks back, he'll see again that the Baltimore aquarium looks very good indeed, a strong, taut, pleasant, ship-shape building in just the right place.