The prime lending rate at most major banks jumped from 20 to 20 1/2 percent today, its second half-point rise this week.

But in the so-called open market -- where banks, businesses and governments raise money directly -- interest rates fell on the news of a small increase in the government's consumer price index and a decline in the federal funds rate, the interest rate that rises and falls in response to the Federal Reserve system's day-to-day monetary policy efforts.

The prime lending rate -- against which most business loan rates are based -- rose today in response to rising costs banks have faced during the last few weeks. If today's decline in open market interest rates continues, then the prime lending rate should decline at some point within the next several weeks.

Interest rates have been rising in large part because the Federal Reserve Board, the nation's central bank, has been tightening its monetary policies in an attempt to hold down the rate of growth of money and credit. Today's reports from the Federal Reserve, however, indicate that the central bank will not find it necessary to tighten further, at least in the immediate future.

The Federal Reserve reported that that money supply (cash and checking account, essentially) declined $2.2 billion last week. During the previous week the money supply grew $3.6 billion. For the last three months, however, the money supply has been growing at a 12.9 percent annual rate, higher than the 10 1/2 percent target rate set by the central bank.

Most economists, including those who set Federal Reserve policy, believe that if the money supply grows too fast it makes inflation worse.

When the Fed tightens monetary policy in an attmept to restrict money growth, it sells government securities on the open market. When banks or dealers pay the central bank, the banks have less funds to make loans.

The Fed also reported today that business loans on the books of the nation's biggest banks declined $569 million last week, compared with a $2.2 billion increase the week before.

The high interest rates spawned by the Fed's policies undoubtedly helped dampen business loan demand. Chase Manhattan, the nation's third-biggest, was the first bank to announce an increase in its prime lending rate, but other banks quickly followed Chase's move. Since the middle of April the prime lending rate has climbed from 17 percent.

Although the prime lending rate is the key charge for most bank loans to businesses for periods of three months to a year, many big corporations can get short-term loans at lower costs, and smaller or less creditworthy companies often pay a premium over the prime rate.

William Sullivan, of the Bank of New York, said that nearly all interest rates declined substantially today, including rates on large certificates of deposit, one of the key sources of funds for big banks. He said the rate on three-month certificates fell to 18.80 percent today from about 19.25 percent Thursday. Six-month rates fell nearly a full percentage point to 17.65 percent.

The federal funds rate -- which banks pay each other to borrow excess reserves and the key rate for Federal Reserve policy watchers -- declined today. The rate averaged about 20 1/4 percent Thursday and about 19 1/4 percent today. The federal funds rate had been about 15 1/2 percent in April when the central bank tightened its monetary policy to choke off what it perceived as an explosion in money growth.

In the bond markets, where prices move in the opposite direction from interest rates, prices rose. Most long-term bonds rose in value about $10 for every $1,000 in face value, Sullivan said.