IN CASE YOU MISSED IT, THE Internal Revenue Service marked its 125th anniversary on July 2. It's too late to send a card, but you can still celebrate by visiting the commemorative exhibition at IRS headquarters on Constitution Avenue, between 10th and 12th streets NW.

The public doesn't seem to be breaking down the doors to pay its respects. On a recent Friday afternoon, the place was deserted, and maybe not just for lack of enthusiasm. A sign at the left of the lobby pointed right to "exhibit entrance." A similar sign at the right pointed left. The entrance in the middle was blocked.

Once you manage to get inside, the mentality that designed the new W-4 form seems in evidence in the arrangement of the exhibit. Viewers are forced to do a peculiar box step as they read the panels from left to right and then follow the exhibit as it runs from right to left. Occasionally you have to switch sides of the corridor without warning in order to follow the chronology of U.S. taxation. Even H&R Block could get addled figuring out where to go next.

The content, though, is entertaining, and the graphics are good. After reviewing the Stamp Act, the Whiskey Rebellion and other great moments in "revenue enhancement" history, we learn that President Lincoln launched an emergency income tax -- and named a commissioner of internal revenue to collect it -- in 1862, in a desperate effort to finance the Civil War. Perhaps fittingly, Abe became one of the first victims of an IRS snafu. With his mind on other things, he sent in two payments on his tax-exempt presidential salary. A mountain of paperwork finally justified a refund, but not until 1872, seven years after Lincoln's death.

As the 19th century ended, random acts of Congress instituted luxury taxes on opium, lawyers and demon rum. Of course, enforcing those taxes created some interesting collection problems. When President Taft brought in the modern income tax in 1913, some of those colorful tasks disappeared, but the exploits of the "revenooers" and Eliot Ness kept the rough-and-tumble traditions of the T-man alive through the '20s. Now, alas, the IRS is a sterile but more efficient mass of computerized data processing.

The exhibit is generally forthright, but it doesn't tell us everything. Its upbeat emphasis on the cheerful compliance of most taxpayers allows no more recent example of tax evasion than Al Capone. It doesn't point out that the IRS imposed 18.6 million penalties amounting to

$3.5 billion in 1986 alone or that it employs more than 4,300 people to ferret out tax fraud.

Although its birthday party is over, the IRS has given you an extension on the exhibit, which runs from 10 to 3 weekdays through October 30. Here's one visit to the IRS that won't end up costing you a cent.