The State Department, housed in perhaps the quintessential government office building of the late 1950s, which means big and pretty dull, has acquired another exquisite suite of 18th-century rooms for its interior.

Just this month Secretary of State George P. Shultz has been able to settle into his small, cherry-paneled study at the end of a sequence of 10 recently redecorated rooms on the building's seventh floor, rooms where much of the nation's high-level diplomacy is carried out.

The rooms were designed in the classical manner by architect Allan Greenberg and furnished mainly with authentic 18th- and early 19th-century American desks, bookcases, tables and chairs, most of them already the property of the department's ongoing Americana Project. Even so, the $1.9 million cost of the job has helped push the privately financed project into sizable debt for the first time in its 24-year existence.

The office part of the project would not have been as large as it was, nor would it have been accomplished last year, were it not for Shultz's enthusiasm, according to Clement E. Conger, chairman of the department's fine arts committee. "We planned eventually to do perhaps four rooms" in the secretary's suite, Conger said, but Shultz insisted on doing all 10. "Which of course is right" from an esthetic point of view, Conger noted, although it did increase costs.

In December 1983, when plans were well under way to convert the Benjamin Franklin State Dining Room into a proposed "classical dining hall . . . which Mr. Franklin might have known," Shultz informed Conger that he wanted his office suite done at the same time to minimize construction noise and disruption. The dining room is located on the eighth floor, directly above the secretary's chambers. During the six months it took to reconstruct his offices, Shultz and his operation moved to another part of the seventh floor. The dining room will be completed by early March.

This was the first time the fine arts committee, which has been receiving up to $1.25 million annually in contributions, had been faced with simultaneously supervising and paying for two major construction projects. Total costs for the two projects, estimated at $3.5 million, actually are running closer to $5.6 million. Despite increased contributions and the sale at auction of numerous antiques it owns (donated objects deemed inappropriate to the period styles of the rooms), the committee will be about $1.2 million in debt when all of the bills come due in February and March, Conger said.

Like the diplomatic reception rooms on the eighth floor, where the Americana Project had been concentrated, the secretary's suite has become a series of museumlike period rooms. It is an odd sensation to enter these rooms from the '50s-modern halls of the original building (even with their occasional classical touches). The first evidence one sees of the change is a big, beautifully crafted wooden architrave that sets off a door in an otherwise ordinary wall of a reception lounge as if it were the grand entrance to a free-standing 18th-century home.

Stepping through the door, one enters a broad foyer with an attractive, arched end wall set off by delicately proportioned, fluted Doric columns. On one side of this foyer is the John Jay reception room, on the other the matching George C. Marshall room -- each a pleasantly scaled place to wait for a meeting with the secretary, each with light-painted wood paneling, Doric pilasters, impressive moldings at the ceiling and, it hardly needs saying, a few museum-quality pieces of 18th-century furniture.

From these rooms one steps into a long, narrow, wainscoted corridor, at either end of which one can look through a keystoned archway, as if from an 18th-century island, back into 20th-century hallways of the building in their unreconstructed state -- another curious sensation. Opening off this corridor are an intimate waiting room with an ingenious built-in corner cupboard, a conference room with a heavy oval table (used at the Williamsburg summit of industrialized nations in 1983) and an impressive formal entryway, and the secretary's offices. These are divided between the study and a large, formal room with moldings even more robustly designed and more impressively crafted than those in the previous rooms. The classical order here changes with a flourish to Corinthian, and the pilaster capitals are embellished with the image of the Great Seal of the United States.

Architect Greenberg says he took inspiration from rooms in existing American mansions, such as the great halls of Stratford Hall in Stratford, Va., or of Cliveden in Philadelphia, but only as starting points for his own inventions. "The grammar and the syntax are the same, but the sentences that come out are different," he says of his attraction to classical motifs.

And it is true that in many details -- such as the way elements of the door and window moldings relate to each other, or in decorative touches (the tiny five-pointed stars in the delicate Doric capitals), or in attempts to hide technology (wainscoting set above stone baseboards that ever so gently expose air ducts) -- the rooms justify Greenberg's confidence that in retrospect "this will be seen just as clearly a product of the late 20th century as Cliveden was of the 18th century." The State Department period rooms are among the best of their kind, and Greenberg's designs for the secretary's chambers are, if anything, a step up in quality over the eighth-floor rooms.