A Metro committee recommended yesterday that free parking be permitted Saturdays at the nine parking lots the transit system operates in an effort to increase use of the subway on weekends. The measure may be voted on by the full Metro board as early as next week.

The lots, built at rail stations to accommodate commuters, are almost fully used on weekdays. But on Saturdays, only 15 to 20 percent of the lots' 5,600 spaces are occupied, according to Metro transportation planner Robert A. Pickett.

Eliminating Saturday charges, Pickett said, would remove a "psychological barrier" to expanded use of trains on Saturday. Most new riders, he suggested, would be tourists or area families on outings.

The proposal was approved yesterday by a 4-to-1 vote of Metro's Revenue and Operations Committee. If the full board endorses the action, the no-fee plan could be in effect by next month.

It would cost Metro about $62,000 a year in lost parking fees (most lots now charge a flat $1.25 a day on Saturdays and weekdays) and would save the transit system about $15,000 in employe wages, according to Pickett.

Pickett said he was not certain whether the proposal would generate 1,500 new trips on Saturdays, the number needed to fully offset the projected less in parking fees. But he said that the experiment should be judged on ridership increases and convenience to the public, as well as cost effectiveness. Parking already is free on Sundays.

Metro's largest lots form part of the stations at New Carrollton (2,148 spaces) and Landover (1,108 spaces). Smaller facilities are located at the Rhode Island Avenue, Fort Totten, Minnesota Avenue, Deanwood, Cheverly, Addison Road and Capitol Heights stations.

In other business, the committee reviewed Metro's quarterly performance report for January through March.

On the positive side, the ratio of Metrol rail's revenue to its operating costs was at projected levels during the period, the report summary said. But Metrorail ridership remained close to last year's levels, failing to register the slight gains that had been projected. Metro spokesman Cody Pfanstiehl said that Metro blames that failure in part on low ridership on the Blue Line from Addison Road.

Increased frequency of breakdowns of automatic Farecard machines also were reported. An average of 85 percent of the machines were functioning at any given time from December through February.

The report said that operating costs of Metrobus operations were running behind projected levels and for the first time since 1978, revenues had covered more than half these expenses. Ridership was keeping pace with projected figures, it said.

However, the report also showed that a higher proportion of the fleet's buses were out of service than was expected, forcing expanded use of reserve buses.