"You don't believe me?" asks the man behind the desk. "Go ahead. Go ahead and lift up my out box."

A vistor does.the desktop shows clear signs of mouse droppings.

"I hate it. I'm really scared about it," says the 18-year-old secretary, on the job just a few weeks. "I've seen five of them. Like this." And she holds her forefingers about four inches apart to indicate the size of the mice that sometimes patrol corridors in the middle of the day.

The setting is not a biology lab gone mad. It is the headquarters of the Environmental Protection Agency, at Waterside Mall, 401 M St. SW.

Staff members say the environment at EPA is so bad as to be a black-humor office joke. The mice, and occasional reported rats, are only the most visible signs of trouble at EPA.Some trouble can be smelled rather than seen.

Five fast-food restaurants are located on the ground floor of the building that houses EPA. Often, the aromas of what they sell seep up in the stairwells, mingling as they go.

The result is a surge of nibbling among some EPA employees, and a surge of disgust in others.

Some security guards say they munch French fires all day, and are glad of the opportunity. But some of their fellow employees don't feel like munching anything. "Have you ever smelled the combination of Italian sausage, fried chicken and souvlaki?" asks one EPA executive.

There is also grumbling about the fast food itself. "I've been here five months and I've gained 11 pounds," says a student intern. "I never want to see another fast-food restaurant as long as I live."

Meanwhile, cockroaches have also been a problem. But all hands say that vermin ans roaches are much prevalent at EPA than they were a few months ago. New traps and poison have been placed by handymen from the General Services Administration (GSA), and a dozen EPA employees interviewed said the new measures have helped.

The question of where EPA employees eat is something else again. EPA workers, their union, their landlord and their boss agree that what is needed is a cafeteria at Waterside Mall.

The lack of a cafeteria stems from the fact that the Mall building was originally supposed to be an apartment complex, not an office building. Thus, according to Charles Bresler, the Mall's developer, space was let on the first floor to a variety of businesses, including the five fast-fooderies.

But since 1975, EPA has occupied all 12 floors of two wings. Approximately 2,850 EPA employees work at Waterside Mall.

By government regulation, they may not take more than 30 minutes for lunch. It would take isolated EPA workers nearly that long just to walk to and from the nearest alternatives.

In addition, many EPA employees consider the surrounding neighborhood unsafe, even by day. So, although some "brown bag it," a majority of EPA employees eat a restaurants on the Mall's main floor.

According to Harold Dodson, president of the Epa local of the American Federation of Government Employees, a union survey taken a few months ago indicted that 90 per cent of the EPA staff would prefer a cafeteria to the current set-up.

The union pressed theissue in late May with EPA administrator Douglas Costle. Costle agreed in turn to pressure GSA, which supervises concessions at all buildings occupied by federal government employees.

Meanwhile, William Maramas, owner of one of the fast-food restaurants at the Mall. The Greek's Subway, applied to GSA last year for permission to turn his place into cafeteria. He proposed to expand from 40 seats to 200, to provide real china and silverware, and to construct whatever sort of menu GSA wished.

Maramas' proposal is now more than nine months old. Debate and correspondence between Maramas, Bresler and GSA continues.

The latest stumbling block is whether Maramas originally proposed to expand to 300 seats instead of 200. No one remembers, or seems to have accurate records. Meanwhile, no cafeteria. And no sure guess as to when one might be expected.

Lurking in the background is a record of unsanitary conditions in two Mall stores, as noted by District government inspectors.

A Safeway grocery store in the Mall, and restaurant that has since folded, where each closed once for sanitary violations, although only for brief periods.

In addition to all the other difficulties at the Mall, Metro construction along M Street has devastated traffic flow and any chance of quiet. "We've had a long, drawn-out bit here," acknowledged Bresler, who announced earlier this year that he does not plan to build additional stuctures on the Mall. About 45 per cent of what he originally planned has been constructed.

One EPA employee said she makes it a point to escape from the Mall for lunch exactly once a week. "I go up to 19th and K and blow $12 on something good," said the woman, a program analyst.

But the other four days, it's Tast food, one flight down - when she isn't jumping up on her chair to evade mice. "You get used to it," she said. "You get used to anything."