The U.S. Treasury Building at the corner of 15th and Pennsylvania NW, built between 1836 and 1869, is the classic power building, whose guards, railings and imposing columns all make the architectural statement: "If you want to come in, bring a tank brigade."

Surprise: You wouldn't know it from checking most Washington tourist guidebooks, but free 90-minute tours of the Greek Revival edifice are offered Saturday mornings (minus the tanks). Not every Saturday, but it's not as complicated as it sounds. All you have to do is call, sometimes only days in advance, to reserve a spot. This is not the place to fulfill fantasy visions of printing presses running off likenesses of dead presidents in their green period. Paper currency is printed at the Bureau of Engraving and Printing, a few blocks south, and the nearest coin mint is in Philadelphia.

But Treasury is nonetheless a fascinating place to tour, a seat of power for the department charged with collecting taxes, advising on monetary, economic and trade policy, investigating and prosecuting tax evaders, counterfeiters, forgers, smugglers, bootleggers and gun law violators, and protecting the president, vice president and their families, among other things.

During the tour you go into working offices, and if you're brazen or tacky enough, you can take a peek at appointment calendars and historic photos, and possibly get some ideas for your next Washington-based intrigue novel. You'll see the sweeping cantilevered staircases, the corner domes, the Washington Monument from the south portico and murals that were once hidden under 15 layers of paint.

You'll see the treasury secretary's conference and reception rooms, refurbished to reflect the style of the late 19th century, lit by Renaissance Revival gas chandeliers and antique white metal and brass wall sconces. You'll learn that there is a tunnel connecting the White House and the Treasury Building, but it hasn't been used since Franklin Roosevelt was in office.

What isn't mentioned on the tour is that the old marble-decorated cash room (site of President Grant's inaugural reception) is where the Senate hearing scene, the last one in the Harrison Ford film "Clear and Present Danger," was filmed.

And there's one more surprise bonus: A 1950 reproduction of the 1776 Liberty Bell on the building's west lawn, between Treasury and the White House. Unlike the Philadelphia bell, you can touch this one.

Tours of the Treasury Building, 1500 Pennsylvania Ave. NW, are given Saturday mornings at 10, 10:20 and 10:40; reservations are required at least three days in advance, and you must provide the name, birth date and Social Security number of everyone in your party. Adults must present a photo ID and children must be at least 12. Video cameras are not allowed. Enter on the lower level (marked Employee Entrance) on the 15th Street side. Metro stops: McPherson Square or Metro Center. For more information visit www.ustreas.gov or call 202-622-0896 or 202-622-0692.

--Judy Colbert, Crofton

The Insider's Guide needs you. Submit items (accompanied by address, daytime phone and Social Security number) via: e-mail (shrodert@washpost.com); fax (202-334-5587); Mail(Insider's Guide, 1150 15th St. NW, Washington, D.C. 20071). No items promoting personal business interests accepted.