Eight of the top 10 commercial banks in the Washington area lowered their prime rates to 9 percent yesterday following the Federal Reserve's decision to decrease the discount rate.

The Bank of Virginia reduced its rate to its best customers by one-half percentage point soon after the major New York banks did so. Throughout the day, Riggs National Bank and American Security Bank in the District; Maryland National Bank; Sovran Bank, Dominion Bank, and United Virginia Bank in Virginia, and the First American Banks of Washington, Maryland and Virginia all followed suit. The cuts were effective immediately or on Monday.

First National Nank of Maryland and Equitable Bank in Maryland maintained their prime rates at 9.5 percent.

The effect on adjustable-rate commercial loans and some consumer loans tied to the prime rate should be felt directly and soon. For other fixed-rate loans that are not indexed to the prime, the effect will vary and may not become apparent for two to four weeks, according to bank officials questioned yesterday.

Several bank officers downplayed the significance of the decline in the discount and prime rates. Charles Duda of First American Bank of Maryland said his bank relies more on an internal cost of funds, such as rates on its certificates of deposit, than on the prime to set other loan rates. He called the discount rate a "political rate."

Alan M. Gayle, an economist for United Virginia Bank, echoed that sentiment.