First, the moral: No threat elicits a faster response within the top ranks of a federal agency than one aimed at its image.

Now, the tale: The Environmental Protection Agency's inspector general checked records pertaining to the agency's 13-car executive fleet and discovered that about one-third of the 3,400 chauffeured trips taken by top EPA officials in a five-month period were to restaurants at lunchtime.

The inspector general, whose report was dated August 1985 but only trickled out of the agency last week, did not suggest that the trips were improper even though the agency's general counsel had ruled in 1979 that government cars were "not ordinarily" supposed to ferry officials to lunch.

But he did note that each trip cost $45 and that the EPA might want to take more cabs, at about $5 a pop allowing for an extra passenger.

The EPA rejected the cab idea, arguing that taxis could not provide acceptable service -- such as mobile telephones that let high-ranking officials stay in touch with the office while mired in traffic.

The question of transportation is sticky at the EPA, which is lodged in Waterside Mall more than half a mile from any other government building. The agency's pesticides division is even farther removed, across the Potomac River in Crystal City, and officials from both installations are on frequent call to Capitol Hill and the Office of Management and Budget.

But EPA skins are thinner than most in Washington, and the agency's lunching habits have been a sensitive subject since the days of Rita Lavelle. So, when the inspector general's report hit print in an Associated Press dispatch, embarrassed agency officials went scrambling for a better defensive posture and some followup figures.

"The real question is whether this agency is entitled to a motor pool for our staff," said Russ Dawson, an assistant to EPA Administrator Lee M. Thomas.

Dawson said the agency has taken steps to cut its transportation costs since the inspector general's audit was conducted in mid-1984, including trimming the executive fleet from 13 cars to eight and restricting the number of officials permitted to use the cars.

Last year, he said, the fleet made an average of 348 trips a month, compared to 683 trips a month in 1984, and only 4 percent of those were to restaurants at lunchtime. Those trips, presumably, were to do business with officials of other agencies, which the general counsel approved.

"There is more efficiency, and the average cost of the trips has been cut by 37 percent, to $28.67 instead of $45," Dawson said.

The agency also has tightened controls on record-keeping and on gasoline credit cards.