About one out of three automobiles that are now being given automobile exhaust tests in the District of Columbia are flunking.

Lawrence Greenberg, deputy assistant director of the D.C. Department of Transportation, who is in charge of the exhaust testing program, said about 125,000 cars have been tested since the program began earlier this year. About 30 to 35 percent failed.

Currently, the exhaust testing is voluntary and there are no sanctions against cars that fail. Mandatory standards and tests will begin next Jan. 1 under federally imposed clean air standards covering Washington and other major metropolitan areas with heavy auto traffic.

Greenberg said the failure rate should drop when mandatory testing begins, since the current voluntary program has higher standards for carbon monoxide and hydrocarbon level than will be required in the future.

The emissions standards vary depending upon the model year of each car. A 1981 or newer car, under the current voluntary program, should have fewer than 300 parts per million of carbon monoxide and less than 1 percent hydrocarbons, or unburned fuel, in its exhaust. The standards are less strict for older cars.

Greenberg said cars that barely pass the current test should be checked before next year. Most repairs are inexpensive, he said, although in some cases costly antipollution equipment in some cars has been found to be inoperative.

Virginia already has begun a mandatory exhaust test for newer model cars in the Washington suburbs. Maryland plans to put a testing program into effect next year.