The National Highway Traffic Safety Administration admitted yesterday that it staged a televised test showing the rear-wheel assembly falling off a 1979 General Motors station wagon.

The film, broadcast April 1 on some television networks, was used in conjunction with an NHTSA announcement that it had found a potentially "catastrophic" defect in as many as 5.3 million midsize GM cars and sedan pickups produced from 1978 through 1980.

The company complained to the NHTSA that the agency did not explain that the filmed assembly failure was simulated. NHTSA's announcement and the film "created unwarranted and unnecessary fears" among GM customers, it said.

NHTSA spokesman Richard Burdette, speaking on behalf of Administrator Raymond Peck, said yesterday that the agency "induced failure" to show what happens when a key part--a C-shaped retaining ring--is removed from the rear axle shaft. The C-clip helps hold the rear-wheel assembly in place.

Burdette said, "The filmed test results released by the agency--and broadcast by some of the news media which covered the agency's actions--accurately, fairly and in accordance with all standard testing practices and procedures portray the consequences of the defect under investigation."

Burdette released an NHTSA "background fact sheet," which said in part: "In all cases, to determine the consequences of a failure requires that a failure be induced. Explosive bolts or other devices to achieve this result under test conditions are commonly used by all test facilities," both in and outside government.

Burdette said the agency described the test as a simulation in a report released on April 1, the day it made its announcement. The report, however, was not widely circulated, NHTSA officials conceded yesterday. And the agency's news release did not indicate that the film was simulated.

But Burdette said yesterday that those considerations were minor, when compared with 144 reported incidents of rear-axle failure in 1978-80 intermediate GM cars and sedan pickups.

"With nobody taking the C-clips off, these accidents occurred in 144 of General Motors' vehicles," Burdette said.

In an April 7 letter to its affected dealers, GM said most rear-wheel assembly systems would fail under the test conditions used by the NHTSA. The company said the 5.3 million vehicles cited by the NHTSA "were driven more than 50 billion miles in the past year" with only 67 incidents of axle malfunction, including 20 cases in which the rear-wheel assembly fell off, resulting in one "alleged injury accident" involving muscle strain.

"This data translates into one such separation for every 265,000 years of vehicle use," GM said.