Peter B. Bensinger, an Illinois Republican who survived the 1977 change to the Carter administration, announced yesterday that he will resign July 10 at the request of the Reagan administration.

"I don't have any sense of unhappiness about the decision," he said. "Each administration is entitled to have its own team."

Criticisms of the federal drug enforcement effort began to surface soon after Attorney General William French Smith got his top aides picked at the Justice Department. With the criticism came renewed talk of merging the Drug Enforcement Administration, which Bensinger heads, into the FBI.

Over the weekend, leaked reports from Justice aides said Bensinger would be fired, citing his attempt to fight budget cuts as proof he wasn't a team player.Others said he was known more for his press release than for managing an agency where a key regional director was accused recently by a federal judge of being a racist.

Shortly after noon yesterday, Bensinger paused to chat with a reporter before hurrying off for an appearance at the FBI Acamedy at Quantico and then a flight to Bismarck, N.D., and another speech Wednesday.

He rattled off statistics of DEA effectiveness with the familiarity of 5 1/2 years on the job, hardly looking or sounding like a man who had announced his forced resignation a few hours earlier.

Bensinger said he agreed that the Carter administration hadn't given a high enough priority to drug enforcement, and he expressed confidence that the new administration would. Reagan officials "have the right legislative agenda and a higher interest. We could see a downtown of [drug] availability. But it's not going to be accomplished with one organizational shift."

He said the newfound interest in the drug problem will have to involve the State Department, Internal Revenue Service, the intelligence community and even the Defense Department, having the Navy help intercept smugglers on the high seas.

Looking back at his tenure, Bensinger noted that though drug overdose death are rising again, they number about 500 a year now as opposed to 2,000 a year in 1976. Heroin purity is up recently as well, from 3.5 percent purity to 3.9 percent, he said. But in 1976 it was 6.6 percent, he said.

The successful program to eradicte opium producing poppy fields in Mexico "must be replicated in Colombia" -- the source of 70 percent of the drugs entering the United States, Bensinger said. Most cocaine and marijuana enters the country now from Central or South America.

Bensinger said "we can finance drug law enforcement out of the proceeds of criminals and make them finance their own demise with increased forfeitures" of seized assets. DEA and Justice were criticized in a recent General Accounting Office report for not aggressively pursuing civil forfeitures.

Bensinger has expressed interest in becoming an ambassador, sources said yesterday, and the administration is willing to look for a spot.