Chemical Bank of New York yesterday lowered its prime lending rate by a full percentage point to 20 1/2 percent, becoming the third major bank in the country to bring down its key lending rate from a record 21 1/2 percent level.

Meanwhile, the Federal Reserve yesterday announced weekly money supply figures confirming a recent slowdown in money growth.The narrow measure of the money supply, M1-A, which is comprised of currency and demand deposits in commercial banks, rose by $500 million in the week ended Dec. 17 to a seasonally adjusted total of $386.9 billion. The second measure of the money supply, called M1-B, went from $412.3 billion to $412.6 billion. M1-B includes cash, checking and savings deposits. The previous week's figures for both measures were revised downward, by $200 million for M1-A and by $100 million for M1-B.

Final figures for this month are likely to show there has been little or no growth in M1-A or M1-B from November's levels. The Fed's vigorous attempts to curb money growth have combined with strong loan demand to push interest rates to record levels this month. A slowdown in money growth should lead to a slackening in interest rates, although few analysts expect rates to drop very far in coming weeks.

Wells Fargo Bank in San Francisco and Chase Manhatten of New York, the nation's third-largest bank, last week cut their prime rates to 20 1/2 percent. But this move was not followed by other major banks. Citibank, the second-largest, announced on Friday that it would keep the prime at 21 1/2 percent for the time being.

Chemical Bank is the nation's sixth-largest. Yesterday's prime rate reduction was in response to a drop in the cost of funds, according to chief economist Fred Deming. But he cautioned that it was uncertain whether further rate declines were likely.

A smaller bank, American National Bank and Trust in Chicago, cut its prime to 20 percent yesterday, and both Chemical Bank and the Bank of New York yesterday cut their broker loan rates to 20 percent from 20 1/2 percent. This is the fee which a bank charges to securities dealers on loans backed by stock as collateral. The prime is the rate charged by banks to their best corporate customers.

Many economists fear that the dramatic rise in interest rates this fall will push the economy back into recession. An economic slowdown is probably already under way and one of the factors moderating money growth.