Civilian employes who hold "critical jobs" at the Pentagon will be tested for drug abuse under a new directive from Deputy Defense Secretary William H. Taft IV.

Expanding a program that is already in effect for military personnel, Taft's directive requires mandatory drug testing for law enforcement officers and those with "jobs involving the protection of property or persons from harm," which apparently refers to persons working with sensitive weapons.

A third category covers "positions involving the national security or the internal security of the Department of Defense in which drug abuse could cause disruption of operations . . . or the potential for unwarranted disclosure of classified information through drug-related blackmail."

Maj. Pete Wyro, a Pentagon spokesman, said he could not predict how many of the department's 1.1 million civilian employes would be covered by the regulation or how much the program will cost.

He said, however, that the program, which takes effect Saturday, will be "limited" and that testing of any employe will have to be "justified on the basis of the job held."

The measure was criticized by government employe unions.

"We're aware of this, and we shudder at it," said Jim Jones, a labor relations specialist with the American Federation of Government Employees. "It's just part of a drift that we've seen into constitutional areas of privacy that worries us." TUBBY TESTS . . .

Another form of testing is being reserved for Air Force personnel considered too fat by their commanders.

An Air Force spokesman said that under a new weight management program, plump pilots will be ordered to undergo tests of their fat-to-muscle ratio and put on diets if it is too high.

The program gives commanders the right to decide whether a pilot presents a "professional military appearance," even if he is below the maximum allowable body weight for his height, according to the spokesman.

A special testing device called a nomogram will be used to gauge the ratio of fat to muscle. According to the nomogram table, men under age 29 can have up to 20 percent fat, with older men allowed 24 percent.

The spokesman said a new table of "desired weights" is about 10 percent lower than the old maximum weights.

"Commanders can even lower those if the member doesn't look good in uniform," he said. "Once someone is placed on the program, he or she will have to lose weight consistently -- and in some cases, faster."

According to the spokesman, fat men will have to lose five pounds a month and fat women, three.

NO HOLDS BARRED . . . One of the hottest lobbying battles in town is between the Northrop Corp., which is pressing the Pentagon to buy its F20 fighter plane, and General Dynamics Corp., which is trying to hang on to prospective orders for its F16 fighter already in the inventory.

On May 17, General Dynamics held what was billed as an urgent meeting with all its subcontractors on the F16 at its plant in Fort Worth. The subcontractors were told that the F20 represented a grave threat to the F16 program, according to executives summoned to the extraordinary session. They were given packets of material showing what business they would lose if the F16 lost out to the F20. The message was that it is very much in their interests to help General Dynamics shoot down the F20, according to participants.

Northrop, meanwhile, is lobbying across a broad front, including seeking the advice of public relations consultants and meeting with editorial boards, in hopes of creating public support for their plane. The company built three F20 Tigersharks, but lost two to crashes. Northrop is expected to handle the remaining F20 carefully until the Pentagon makes an up or down decision on the fighter.