MEXICO CITY, FEB. 10 -- This weekend President Reagan is to visit the home turf of Mexico's major narcotics bosses, a sign of guarded U.S. approval of new Mexican efforts to fight drug-generated violence and corruption there.

For his sixth and probably last official meeting with Mexican President Miguel de la Madrid, Reagan will fly Saturday morning to Mazatlan, a balmy Pacific beach resort popular with college-age Americans.

The largest city in Sinaloa State, Mazatlan has long been a business and recreation center for the Sinaloa-based gangs who dominate Mexico's narcotics trade. Eight U.S. Drug Enforcement Administration agents operate out of the heavily guarded U.S. Consulate there, tracking the movements of narcotics bosses who Mexican sources acknowledge had corrupted local police and turned Mazatlan into a drug runners' haven.

The Sinaloa traffickers also have been charged with the kidnaping and murder three years ago this week of DEA agent Enrique Camarena Salazar, which produced a sharp rift between the Reagan and de la Madrid administrations.

Mexico proposed Mazatlan after the White House requested a summit venue close to California, where Reagan is to spend Saturday night, diplomats said. But if Mexico had suggested a presidential meeting meeting in Mazatlan as recently as a year ago, the administration "would probably have asked for it to be held someplace else," one U.S. diplomat said.

Washington's view of Sinaloa has changed since Francisco Labastida Ochoa, a former energy minister, became governor in January 1987. He is being praised by U.S. and Mexican authorities for reversing what they said was a pattern of official tolerance of the drug lords.

"Labastida is doing a tremendous job," said a U.S. official close to the DEA. "The narco gangs used to be able to count on the governor's services in Sinaloa. Now that system is breaking down."

The state's narcotics trade remains entrenched, however, Mexican and U.S. officials acknowledge. Sinaloa has been Mexico's main drug-trafficking center since the 1950s, when the country began exporting heroin and marijuana.

After two decades of U.S.-financed crop destruction campaigns, Sinaloa's sparsely inhabited sierra is still dotted with poppy fields. Scores of remote airstrips also have helped make the state a major transshipment point for U.S.-bound cocaine, some of which has been processed in Sinaloa's clandestine laboratories.

Although several drug kingpins have been caught and imprisoned, Miguel Angel Felix Gallardo, reputedly the top Sinaloa narcotics boss, is at large. He is said by DEA and Mexican law enforcement sources to have masterminded a worldwide cocaine distribution ring and is believed to have ordered Camarena's killing and to have escaped arrest with the aid of officials in Sinaloa's previous government.

"Felix Gallardo is still active but not on the same scale as before," asserted one well-informed source. "He used to run on average two loads a month, a $30 million-a-month operation. Now he might run a load every two or three months, and he knows that the state is not on his side."

One measure of this change, observers said, is a notable downturn in violence. Labastida pointed proudly in a recent interview to the state's record of 400 homicides last year, down from 1,400 in 1986, when the state's per capita rate was Mexico's highest. His revamped police force has confiscated hundreds of unregistered weapons, many of them powerful automatic rifles smuggled from the United States. He fired 1,300 state security officers and jailed 100 more on corruption charges, withstanding what Mexican sources said was intense internal police pressure.

"If there isn't any resistance, then you aren't doing it right," Labastida said.

To aid Labastida, the federal government has bolstered the troop strength of the military district that embraces Sinaloa and appointed aggressive new commanders. The local Army detachment is assigned almost exclusively to narcotics control.

The new Army officers and the new governor "were sent to Sinaloa as a package, and the message was that the federal government was not going to tolerate the traffickers any longer," a U.S. diplomat said.

Many U.S. officials remain critical of what they say is Mexico's failure to pursue vigorously local drug chieftains and to punish allegedly corrupt former officials, such as former Mazatlan mayor Antonio Toledo Corro. "Labastida is making a good-faith effort, but Sinaloa is still a serious problem," a U.S. government spokesman said. "He may have gone as far as he can go with state resources. He wants and needs more aid from the federal government."

Moreover, some assert, the crackdown in Sinaloa has simply displaced drug cultivation and transshipment operations to other states, sometimes with official complicity.

Despite their dissatisfaction with Mexican antinarcotics efforts generally, U.S. government officials directly involved with drug enforcement here contend that the Sinaloa shake-up is symptomatic of what one called "a new seriousness" in Mexico's drug control campaign.

One of the few federal budget items not cut in a new austerity drive, the officials note, was the attorney general's allocation, about 60 percent of which is spent on narcotics enforcement. They also point out that 15 Mexican state and federal security officers were killed last year in confrontations with traffickers.

Critics argue that the government is committing too much of its scant resources to fighting a problem that is ultimately attributable to drug consumption within the United States.

Attorney General Sergio Garcia Ramirez, who will meet Saturday in Mazatlan with his counterpart Edwin Meese III, expressed frustration last week with what he says is a U.S. "lack of appreciation of how difficult it is for Mexico, in its difficult economic circumstances, to fight this battle against drugs."