AT SOME point, an optimistic observer might have assumed that racial discrimination in rental housing in the Washington area was on the decline. Congress had finally added teeth to the enforcement of federal fair housing laws. Lawyers had successfully sued several real estate agents, landlords and apartment owners for racial bias in several highly publicized cases in recent years. But the Fair Housing Council of Greater Washington has conducted more than 1,000 field studies throughout the past five years that highlight the continued existence of prejudice against blacks.

In fact the results of the Fair Housing Council's most recent year of work, released this week, show the highest level of racial discrimination ever recorded. The council again used the court-approved process of "testing," sending blacks and whites with similar status to seek apartments at some 200 rental properties. In nearly two-thirds of the cases, blacks were discouraged from applying for apartments, and whites were given preferential treatment.

That discouragement took several forms. Blacks were told that apartments were only available at a later date, at the same time that whites were told units were ready for immediate occupancy. One black woman was told to go home and think about whether she really wanted to live at a certain apartment complex. A white tester at the same property was shown three apartments and all of the complex's separate amenities. In other instances, blacks were quoted higher application fees and security deposits than whites and were not told of special incentives, such as a free month's rent or the fact that no security deposit was required.

Montgomery County turned in the worst performance this year, with preference shown to whites 70 percent of the time. The comparable figure was 62 percent for both Northern Virginia and the District of Columbia, followed by Prince George's County, where the least amount of discrimination was found.

The results show that a distressing level of racial prejudice persists. The results also show that tough enforcement of fair housing laws was never more important in the Washington area and should continue with added vigor.