Though showing certain signs of wear, the Organization of American States building, dedicated in 1910, heads into its 75th year very much the way it started out, which is to say it remains one of the more beautiful, restful and interesting buildings in Washington.

The building is one of the chief accomplishments of the City Beautiful movement in a city with an abundance of them. It sits just so on its green site at the junction of 17th Street and Constitution Avenue NW, inviting entry even as it commands respect. Once entered, it welcomes the visitor in a sequence of spaces that is both intimate and grand. Throughout, the program of ornament excites admiration for its elegance, craft and ingenuity.

In the years following the release of the McMillan Commission report in 1902 there was a flurry of construction in Washington. Union Station, the District Building, the Cannon and Russell congressional office buildings, the Agriculture Department, the Natural History Museum, the Army War College and the Memorial Continental Hall of the Daughters of the American Revolution (which is next door to the OAS building) were completed between 1907 and 1911.

As Constance McLaughlin Green noted in her history of Washington, this impressive list of Neo-Classical buildings "immediately added to the city's aura of dignity." And yet with the exception of Daniel Burnham's Union Station, which was altogether a different, larger scale of enterprise, no structure on the list remains as appealing in so many ways as the OAS.

From start to finish the OAS was designed with the flow of people in mind. Its architects, Albert Kelsey and Paul Cret of Philadelphia, were relatively inexperienced when they won the competition for the right to design the building, and they were intelligent enough to change many aspects of their entry as construction deadlines approached. But their earliest free-hand sketches foresee the orderly sequence of spaces that remains one of the building's principal delights.

Today, just as the architects first imagined it, a visit to the building begins in a spacious, table-like forecourt facing 17th Street and continues up broad entrance stairs and through arched doors to a handsome vaulted vestibule. From there, visitors have an enchanting choice: They can pass directly into a skylit patio and progress to a sunken gallery (originally the library reading room) or they can climb either of the wide stairwells flanking the patio and progress to the vaulted assembly hall, one of the brightest and biggest public rooms in all of Washington. From this hall on special occasions guests can exit, again on beautifully planned ceremonial stairwells, into the Blue Aztec Garden, one of the city's more beautiful outdoor "rooms."

The forthright logic of this progression is typical of its time -- the plan has "Ecole des Beaux Arts" written all over it. Thus it is no surprise to learn that Cret (whose later contributions to Washington include the Folger Shakespeare Library and the Federal Reserve building) had graduated from the famous French architectural academy in 1903, only four years before collaborating on the OAS entry, and that Kelsey had studied in Paris after his graduation from the University of Pennsylvania in 1896. But far from being a dry academic exercise, their graceful plan provides the armature for truly engaging spaces.

From the beginning, Kelsey and Cret were aware that the building had to serve several masters -- to be at once North American and South American in character, and to harmonize with the "generally classic or Renaissance" quality, as an early director put it, of monumental buildings in Washington. That they managed to make a unified whole out of such diverse elements is a great tribute to their skill, and also to their flexibility. Under the careful oversight of Charles Follen McKim, one of the authors of the McMillan Commission report and in many respects the preeminent American architect of the time, and with the assistance of a dedicated group of enthusiasts for the project, the pair made many a change in their competition entry. These ranged from minor to crucial.

Originally, for instance, they had planned a "monumental fence and gate" standing free in front of the main entrance. "Here was a Monarchical idea as opposed to a Democratic idea," they wrote, and the fence became the Spanish bronze grilles more politely placed under the glass fanlights of the arched entryways. More critical was their dawning realization that the patio, as initially designed, was "as much Italian as Spanish" in character. They changed the design to its present, lovely state, remindful of the types of shaded, enclosed courts one sees in cities throughout Spain and Latin America. Even after repeated visits this shift in architectural character, subtly heralded in the few Spanish touches on the outside of the building, retains its capacity to surprise and please.

Excepting perhaps the first of the Library of Congress buildings, completed in 1897, no building in Washington is so notable an example of the unity of architecture and ornament than the OAS, and no other ornamental program is so complex in symbolism. Almost every place one looks outside and inside this building -- from the sculptural groups representing North and South America flanking the entrance, to the fountain and tile floor in the patio, to the Tiffany windows in the assembly hall -- there are reminders of the North-South, New World-Old World heritage of the building and the organization it houses. This richness of detail and symbolism contributes no end to the enormous likability of this building after 75 years.

As for those signs of age, they are plentiful: missing tiles in the Aztec garden, absent pieces in the front-door grilles, acid-rain damage to exterior stonework, broken sculptural noses, Tiffany windows obscured by air conditioning units. But they are under the watchful care of Larry Woods, a building superintendent who bows to no one in enthusiasm for his building, and they are, all things considered, minor.

Thanks to Woods' attentions, and above all to Kelsey and Cret, one of the nicest things one can do on a sunny Washington afternoon is to spend some time on the main stairwells inside the OAS, with views of the patio, heavily shaded with tropical vegetation (though, alas, absent the tropical birds that lived there for many years), and of the richly ornamented stucco ceiling of the assembly hall, brightly illumined by the western sun. After next Thursday, one can also go there to enjoy an exhibit of photographs commemorating the building's anniversary.