The President's Commission on Organized Crime began hearings here today on heroin distribution in the United States, taking testimony from U.S. and Italian officials and a former Mafia "hit man" and heroin distributor.

John C. Lawn, deputy administrator of the federal Drug Enforcement Administration, said the amount of heroin imported into the United States has remained fairly constant recently, with 4.2 tons to 4.5 tons imported last year. The estimated number of users has also remained constant, he said, at about a half million.

Michael J. Tobin, who heads DEA's heroin investigation section, said in written testimony that Mafia families have been involved in heroin distribution in this country since the late 1930s.

"Today there are 26 traditional organized-crime families operating in the United States . . . involved in the whole spectrum of drug trafficking and in all types of illicit drugs," Tobin said.

He added that Italy is the major trafficking country for heroin being transported to Europe and North America from Southeast and Southwest Asia.

Lawn estimated that 17 percent of the heroin entering the United States is from Southeast Asia, 49 percent is from Southwest Asia and 34 percent from Mexico.

Three Italian officials -- Questore Umberto Improta of the Italian Commission for Mafia Affairs, Antonio Ragusa of the national police, or carabinieri, and Lt. Col. Giovanni Monaco of the treasury police -- testified about efforts to end the heroin trade in Italy, including a law enacted in 1982 making it a crime even to associate with known Mafia figures.

The Italians also entered into a new extradition treaty with the United States last year to curb the drug traffic.

Improta testified that three organized-crime groups in Italy are active in heroin trafficking: the traditional Mafia, which is based in Sicily; the Naples-based Camorra, and the Indrangheta, based in Calabria.

The commission took testimony today from Sicilian-born Luigi Ronsisvalle, who settled in the United States in 1966 and is currently serving a 5- to 15-year prison sentence on convictions for murder and threatening a witness in a federal trial.

Ronsisvalle, 44, detailed his activities as a Mafia hit man and heroin courier for the New York-based Bonanno organized-crime family before his 1979 arrest. He said he carried out 13 contract hits and delivered hundreds of pounds of heroin for the Bonanno family to contacts in New York, Chicago and Los Angeles.

Testifying under oath, Ronsisvalle said that in late 1978 he turned down a $100,000 contract to kill an unidentified assistant U.S. attorney in New York involved in prosecuting the Michele Sindona case. Sindona is serving a 25-year prison sentence in connection with the collapse of the Franklin National Bank and has been extradited to Italy to face other charges.

Ronsisvalle said that as a child in Sicily he longed to join the Mafia. "Like an American kid who falls in love with baseball, I fell in love with the Mafia," he said.

But his life as a Mafia operative ended abruptly in 1979, he said, when he robbed a female payroll courier and was caught by police.

"The worst thing a man can do is rob a lady," Ronsisvalle said. "When I did that, I lost all my honor."

Unable to get his friends or relatives to post his bail because he had "lost face," Ronsisvalle said he tried to extort the money, and then turned himself over to the Federal Bureau of Investigation in desperation.