Go to the restroom, flush the john and wash your hands. Go to the cafeteria and have a cup of coffee. But don't try to slake your thirst from the drinking fountains.

That's the official word for the James Madison Memorial Building of the Library of Congress. Drinking fountains have been turned off since a week ago Monday because the water looked and tasted awful. In the bureaucratese of the Library Support Services Office, "the water . . . might have become contaminated and not suitable for consumption."

Or, as signs on the fountains put it, the water was not "potable" -- a word whose understanding, one library employe noted, requires a fairly sophisticated English vocabulary for the library's many young and foreign visitors.

Some people continued to drink, despite the signs, so the library finally cut off the water.

What had happened, according to library spokeswoman Nancy Bush, was that a wrong type of filter was installed in Madison's chilled-water system, causing a bad taste. Seeking to resolve the problem, employes of the Architect of the Capitol -- whose office maintains the library complex -- decided to flush the lines. That stirred up a lot of accumulated sediment. Yecch!

Water samples were checked by the D.C. government and found to be medically safe if unpalatable. A second set of samples was sent to a private laboratory in Rockville, but the results were not available as of yesterday afternoon.

Meanwhile, Bush said, the fountains remain disconnected, but water in the cafeteria and restrooms -- supplied by other pipe systems -- is safe. No Sign of a Snake

It sounds like a combination April Fools' Day spoof and a barb at a senatorial critic of President Reagan's policy toward Nicaragua.

Signs popped up around Reston on Tuesday (April 1), according to Fairfax County police Capt. R.J. Rappoport, that proclaimed the loss of a "32-foot Anaconda snake, dark olive-brown color with large oval black spots along the back . . . extremely dangerous!! If seen, do not attempt to capture!! Call 224-3344, Mr. James Sasser."

Post reporter Patricia Davis tried to reach Sen. Sasser (D-Tenn.) at the Capitol number indicated on the sign, and the phone rang and rang.

"I'm calling about the snake," she said when a woman finally answered. "Oh, yeah, you need the press spokesman," the woman said. But the spokesman never called back.

And, Rappoport said, nobody ever claimed responsibility for the signs. The First Grandchild

Stanley First, a Washington lawyer and acquaintance, often wonders teasingly when Metro Scene will print an item about him. My stock answer: "When you're first in something."

Yesterday he walked into the neighborhood restaurant and announced the first First grandchild, thus meeting the prerequisite for an item. His and Faye First's son David and daughter-in-law Nancy, of Columbia, are the parents of Michael, born Monday at Sinai Hospital in Baltimore. To get another such item in, Stanley will have to change his surname to Second.