First American Banks of Virginia, Maryland and the District had their certificates of deposit downgraded yesterday by Standard & Poor's Corp. because of the rating company's concern over the banks' exposure to the area's sluggish real estate market.

"The downgrade reflects deteriorating asset quality, the risk inherent in the banks' loan portfolios due to a high proportion of construction and commercial real estate credits, and weak earnings prospects," Standard & Poor's said.

The downgrading is the latest in a series of similar actions against local financial institutions. Perpetual Savings Bank, Sovran, Maryland National Bank, American Security Bank, Baltimore Bancorp and Riggs all have had their debt ratings lowered in recent months by Standard & Poor's and Moody's Investors Service.

The changes mean that the credit rating agencies believe the banks' debt or certificates of deposit have become more risky investments.

Standard & Poor's lowered the rating of First American CDs one notch to BBB--A-3 from BBB+-A-2. The Standard & Poor's rating scale ranges from AAA to D.

Standard & Poor's spokeswoman Sheila D. Davis said the firm is concerned about the slowdown in the Washington area real estate market and "expects problem real estate assets to continue to increase in 1990."

She said she expects the profitability of many Washington area banks to be "at a low level over the near term" because the banks will need to add more cash to their loan loss reserves, the cushion that protects against any deterioration of asset quality.

Many area banks increased those reserves in the first quarter and, as a result, saw a drop in earnings or a loss. Davis said she expects a similar performance by banks in this area and along the East Coast in the second quarter, which ends next week.

"We've already watched the nonperforming loans go up and asset quality deteriorate," she said. "And as the real estate market continues to soften, we know nonperforming loans will continue to rise."