President Reagan, asked tonight about the controversial MX missile, told cheering Republicans at a fund-raiser, "I don't know where we're going to put it, but we're going to have it."

The president quickly followed by saying that " a decision has not been made yet" on various strategic options for basing the missile.

Reagan, who has become fond of such quick give-and-take sessions with admirers, seemed to be in an expansive mood after another week at his mountaintop ranch. Tonight's $250-a-plate reception was held at an estate in northern Santa Barbara.

In other comments, Reagan sniped at the Federal Reserve System for maintaining high interest rates, reminisced about the gold standard and needled California Gov. Edmund G. (Jerry) Brown Jr., a Democrat, for his handling of the Mediterranean fruit fly crisis.

When a women in the crowd of 1,200 identified herself as a real estate agent and asked about high interest rates, Reagan said: "The Federal Reserve System is independent, and they are hurting us in what we are trying to do as much as they are hurting everyone else."

The president said consumers must suffer high rates as long as the federal government borrowers heavily to finance a large deficit but said he hopes rates would "be coming down before the end of the year."

After the session, deputy press secretary Larry Speakes took pains to emphasize to reporters that Reagan's remarks should be interpreted as criticism of interest rates, not the Federal Reserve.

Another questioner asked if interest rates might come down to 3 percent if the United States returned to the gold standard.

"I'm old enough to remember when we were [at 3 percent], and we were on the gold standard," Reagan said. Then he quickly added, "I don't have the answer to that" and noted that a commission is studying possible effects of a return to basing paper currency firmly on gold."

Referring to the Medfly crisis, Reagan said: "I remember when I was running for governor the first time, and they were having trouble in the orchards then. It was called 'Brown rot,'" an apparent reference to Brown and Brown's father, who was the incumbent Reagan defeated in the governor's of 1966.

Asked about funding for the Medfly spraying progra,, Reagan said: "It is a very serious problem, and maybe the problem was we didn't spend the money soon enough."

Brown recently ordered aerial spraying of produce fields but only after federal pressure to do so.

Reagan complained about premature news accounts of decision on the MX system. Asked to describe his biggest surprise since taking offce, he said:

". . .the leaks. It's gotten so I address some things in the Cabinet meetings to the chandelier. I'm sure it must have a microphone in it. . . . We haven't even made a decision, and we're reading in the paper what the decision is supposed to be."

Then, to resounding applause, Reagan added: "I guess the other surprise is, to tell you the truth, after all the horror stories about the job, I'm kind of enjoying myself."

He says he wants 1982 to be "the year of the elephant." Supporters cheered as he said his budget cuts "will leave in the next three years more than $38.5 billion in the hands of the people of California to spend as they would spend it rather than as the federal government would spend it."

Reagan told the group he has received a morale boot last week on the aircraft carrier Constellation. There, he said, a sailor told him: "We may not be the biggest navy in the world, but we're the best."