BOGOTA, COLOMBIA, DEC. 18 -- Fabio Ochoa Vasquez, under indictment in the United States for drug trafficking and planning the murder of a key Drug Enforcement Administration informant, today surrendered to Colombian authorities under a government decree guaranteeing that he will not be extradited to the United States.

Ochoa, 33, and two older brothers are on the DEA's list of 12 most-wanted Colombian cocaine traffickers, and he is a reputed leader of the Medellin drug cartel. As part of the government decree, he is to receive reduced sentences for any crimes to which he confesses.

Ochoa faces no criminal charges in Colombia but is under a U.S. federal grand jury indictment for helping smuggle an estimated $1 billion worth of cocaine into the country and for planning the 1986 murder of DEA informant Barry Seal.

"The government is very satisfied that the decree . . . is producing all the effects we wanted it to," President Cesar Gaviria told reporters following the announcement. "The decree was to allow delinquents to submit to Colombian justice."

Ochoa, allegedly one of the pioneers of large-scale cocaine trafficking to the United States, is the first prominent trafficker to turn himself in since a Sept. 5 announcement by Gaviria offering incentives to traffickers who surrender. Five traffickers so far have turned themselves in.

According to a communique released by the Ochoa family, the youngest son turned himself in to Judge Marta Luz Hurtado at midday at the main church in Caldas, 12 miles south of Medellin. Under very tight security, he was taken to an undisclosed prison in Medellin after giving his statement to a judge.

Ochoa is alleged to have spent time in Miami, managing the cocaine business, and in Portugal, training to be a bullfighter, his long-time ambition.

The maximum sentence for any crime in Colombia is 30 years in prison, and under the decree, if Ochoa fully confessed and halted his drug operations, he could be released in eight years, senior government officials said.

Major Colombian drug traffickers, who call themselves the Extraditables because they are wanted in the United States, have been carrying on a series of public communications with the government, offering to turn themselves in and halt terrorist and trafficking activities in exchange for guarantees that they will not be extradited and will be imprisoned in specially furnished facilities.

The United States has long viewed extradition as necessary because the traffickers for years have killed, intimidated or bribed judges handling sensitive cases in Colombia.

The decree and subsequent modifications, including the promise of special prisons, has been criticized internationally because it appeared the government was softening its position on the drug war and because many law enforcement officials are skeptical that the traffickers will give up their business.

"This shows our policy works," said a senior government official. "The policy is succeeding, and we have the right to be happy."

But an international narcotics expert was less enthusiastic. "The policy will work only if he gets 30 years in jail," the official said. "If he is out in two or three years, it will be a farce."

Under the decree, Ochoa will have to confess to at least one crime that allows him to be jailed in Colombia. The United States will then have nine months to present evidence proving that he committed other crimes in hopes of extending his prison sentence.