Sales of previously owned homes fell 4.5 percent in January to the lowest level in 21 months as high mortgage rates locked more prospective buyers out of the market.

The National Association of Realtors said sales of existing single-family homes totaled 3.59 million at a seasonally adjusted annual rate, down from 3.76 million in December.The January rate, lowest since 3.46 million in April 1993, also was 13.9 percent below the 4.17 million pace during January 1994.

Sales had risen 1.9 percent in December, the first advance since last March, according to revised figures released by the Realtors. But they resumed their fall in January, with the Northeast posting the only regional advance.

According to the Federal Home Loan Mortgage Corp., fixed-rate, 30-year mortgages averaged 9.15 percent in January, up sharply from 7.07 percent in January 1994.

An increase from 7 percent to 9 percent would add $139.32 to the monthly payment on a $100,000 mortgage.

"There's no question that high interest rates have priced out some buyers," said Edmund G. Woods Jr., the association president.

Realtors' projections suggest further declines this year.