One doesn't need or necessarily want to think much about certain streets in downtown Washington, especially in the western sector rebuilt almost completely during the '60s and '70s. Framed by 10-story office buildings, these streets are what passes for canyons in the low city, which is to say they're well defined but otherwise without redeeming feature. One registers them as blank passageways from one place to another.

Such a neuter until recently was the 1100 block of 18th Street NW. Bounded by indifferent or even awful buildings, its sidewalks marred by hard toothless mouths leading to underground parking garages, this block was of no interest except as a conduit to livelier destinations nearby. It has perked up quite some, however, with the arrival of a bright mid-block building designed by the Boston architectural firm of Don M. Hisaka and Associates.

Unusual in this of late architecturally conservative town, the building at 1150 18th St. NW is unapologetically a modern one, at least at first glance. "We were getting tired of looking at fake Mansard roofs," says Hisaka aggressively, although he hastens to point out that there are no historic buildings in the immediate surround. "Heavy" is Hisaka's preferred word to describe the existing architecture -- an appropriate, terse description for the more-or-less modern buildings that make up the immediate context for this, his second Washington effort for the Kaempfer Co.

Hisaka's design, then, is less a shift of style than a purposeful change of color, materials and pace. It is white, in contrast to the prevailing tones of brown; it's all sharp metal and clear glass, instead of granite or precast concrete and tinted windows; and for all the symmetry of its relatively narrow facade, its rhythm is adamantly staccato in comparison to the bland, "tasteful" cadence all around.

Squeezed by the nearly unrelenting bulk of the structures to the north and south (1800 M St. and 1801 L St., respectively), the new building is like an exclamation point that fits somehow in the middle of a ponderous sentence. It's a sliver that attracts the eye from a distance. The contrasting color, intended as a playful recall of the city's famous "white" monuments, would alone assure this response; the pair of slender metal flag-flying parapets simply adds to the effect.

Things get curiouser and curiouser the closer one gets. Architecture buffs will ask themselves: Is this a modern building, or what? The answer they'll come to is: It is and it isn't. It is modern in its self-conscious singularity, its materials, its abstraction, its overall image. It is postmodern in its layering of references and its willful symbolism: The building identifies itself with a mandala-like metal "sign" centered high on its facade. And it's possibly a bit anti-modern in the way it twits solemnity.

This is serious architecture, sure enough, but it also is a funny building -- funny ha-ha, funny strange. The herky-jerky syncopation of the facade is pretty wild. It's as if Hisaka took a lesson from the master, Mies van der Rohe, on how to use metal members as ornamental devices and then turned Mies's matte-black steel into gleaming white aluminum, his verticality to horizontality, his solemnity to insouciance, his sonorous regularity to a jazz beat. Study this facade for a moment: Your eye won't find rest among all the little visual pulsings.

Purists of any persuasion won't like this unrestful product, but most everybody else will, for the simple reason that it's oddly likable: It has pattern, shine, texture, elan. For a curtain-wall facade, this one even has a certain appealing physical depth -- a lesson many modern architects forgot while pursuing the romance of the shimmering all-glass plane. Most important, perhaps, this building pays careful attention to the needs of the street. It is pleasant to pass and to enter and, once entered, surprising.

The site -- narrow on the street, expanding in a L-shape to the rear -- was ingeniously exploited by the architect. Getting light to the office interiors in the back was a prime consideration. Hence the long, alleylike, single-story hallway leading to an atrium with a south-facing wall made mostly of glass.

In itself, this is an effective, closed-to-open sequence of spaces. Hisaka reinforces the sequence with a bit of low-budget decorative razzle-dazzle -- exposed trusses, mirrors, playful contrasts of stonework with wallboard and various metals, industrial-type lamp fixtures customized from standard components. This is a kit of parts he introduced to the city three years ago in the interior of his first Kaempfer project, the office building at 1250 24th St. NW. Here, as there, it makes for a quirky look, combining erector-set homeliness with the austerity of, say, a Richard Meier.

Equally as important to the mood of the place -- more so, probably -- are niceties of the design in floor plan and cross section. There are playful minor touches and critical major ones. Exposed concrete columns in the atrium, for instance, are treated as an occasion for useless but appealing little window-glass enclosures; a semicircular balcony cantilevered into the atrium brings the entrance hallway to an appropriate termination and, as echoed in smaller balconies on the upper floors, provides tenants with dizzying overlooks.

As in his previous D.C. building, Hisaka incorporates the basement level into the overall space. It's a tactic other architects and developers ought seriously to consider, for it appears to work, adding not only visual but also economic value to the building. Because of the small floor area available, this atrium is something of a vertical tunnel. But, like much else here, it's engaging in a vive-la-difference way: Necessity begets invention. With more glass per square foot of surface than any other office atrium in the city, this is one bright tunnel.

The architect's cleverest move by far was to enclose the atrium above the fifth floor, leaving only an off-center, pyramidal glass opening in its ceiling. Besides further reducing the tunnel effect, this enabled him to create two unusual, open interiors on the upper floors, two and three stories high, respectively. These are genuinely stunning spaces, and useful too: It is not at all surprising to learn that, despite having just about anywhere to choose from in today's office glut, a local law firm snapped up the top three floors on the strength of this design.

A couple more things deserve mention. Reaching for every available ray of light and the minutest view, the architect fully designed the alley elevation of this building, a praiseworthy touch whether seen from inside or outside. Likewise (and credit here the client too), he made appealing use of the rear facade: big, handsome, white-mullioned windows punched in a gray-and-white patterned wall. As office workers in downtown west well know, part of the genius of the place is that alley interiors of its blocks often are more lively than the street frontages. This block follows the rule: The new building will allow another restaurant to open onto an urban cul-de-sac, joining two already in existence there.

Weaned on modernism like most architects of his generation (he received his master's degree from the Harvard Graduate School of Design in 1953), Hisaka, unlike some, never quite gave up the modernist ghost. After founding his own firm in the 1960s, he turned out many a minimal, monumental, quasi-logical, weirdly geometrical schoolhouse, library etc., and his abiding concerns continue to be with light, color, abstract space and abstract ornament. But his architecture has been at once chastened and enriched by the revisionist debates of the past two decades. Or perhaps it is just a wise sort of effervescence attending age that we see in the curious, contrasty, poignantly powerful commercial buildings he has designed in Washington.