Falling mortgage rates lighted a fire under existing home sales in Baltimore in December, as the Greater Baltimore Board of Realtors said yesterday that sales jumped 34 percent from December 1990.

Experts discounted the increase slightly because December is always a slow month for real estate because of the holidays, and December 1990 was off 26 percent from December 1989 because of the tensions in the Persian Gulf region. But they hailed the news as a sign that local housing may be starting to shake off its slump.

"It does look wonderful because last year is so bad. But it was good even compared to earlier Decembers," said Nancy Hubble, vice president of the Board of Realtors. "Since Jan. 1, all the [real estate sales] offices I've talked to have been frantically busy."

"It's the right direction, and at this point that's what's important," said David Donabedian, an economist for Mercantile Safe Deposit & Trust in Baltimore and himself a brand-new homeowner. "We've had a couple of years of stable prices and falling interest rates, and that's a potent combination for affordability."

The Realtors' figures track contracts of sale for homes in Baltimore and in Baltimore, Carroll, Harford, Howard and Kent counties. Anne Arundel County has its own Board of Realtors and releases its figures separately.

In the area, consumers contracted to buy 927 homes in December, up from 690 in December 1990. Settled sales rose 15 percent, reflecting market activity earlier in the fall. Settlement statistics typically trail contracts by about two months, reflecting the time it takes for buyers to line up mortgages and arrange other details before closing.