The federal employee drug-testing program instituted by President Reagan will quadruple in size next year unless stopped in its tracks by legal challenges.

An estimated 50,000 employees were tested this year, mainly law enforcement officers, transportation inspectors and civilians working for the military.But the program is expected to test some 200,000 members of the 3.1 million federal work force in 1989, according to the National Institute of Drug Abuse.

About 150,000 to 170,000 of those will be tested at random and come from a pool of some 500,000 workers in positions deemed sensitive by their agency chiefs.

The remainder will be tested after accidents or when supervisors believe there is reasonable suspicion of drug use.

Those figures do not include untold numbers of federal job applicants who will be tested if applying for sensitive positions, nor do they reflect the Transportation Department's far-reaching requirement to test nearly 4 million private sector transportation workers - from truckers to airline pilots.