Four years after a largely U.S.-financed anti-drug campaign dried up much of this country's illicit narcotics trade, Mexico again has become a leading U.S. source of heroin and marijuana and conduit for much of the South American cocaine headed north.

In response to the rapidly escalating traffic, the Mexican government has launched a major anti-drug assault that is turning some areas of the country into battle zones, where skirmishes with the growing drug "mafia" already have cost the lives of scores of Mexican security officers.

U. S. drug enforcement officials have been working closely with the Mexicans. U.S. agent Enrique Camarena Salazar, who was abducted in Guadalajara last Thursday, is now feared dead. If so, he would be the first American casualty of the stepped-up battle.

"This is a war," said Jesus Antonio Sam Lopez, chief of Mexico's Narcotics Control Directorate, who reports losing seven of his several hundred agents in clashes with traffickers last month alone. A dozen soldiers were also reported killed in anti-drug actions in recent weeks.

Since the Mexican government began its campaign against narcotics a year ago, 100 police or military men are believed to have died. "The casualties are high, but we don't like to talk about them," said Sam, a 25-year police veteran with a law degree who took charge of Mexico's anti-drug drive after the change of administrations here in late 1982.

"We altered our strategy," Sam explained, with agents now concentrating their forces in coordinated blitz-like attacks on target cultivation zones, an approach that offers both high rewards and high risks. "Instead of using 40 troops like we did before, we might use 400."

Mexico's current government "has put a lot of resources into the drug campaign, and it has lost a lot of people," commented a U.S. official familiar with the anti-narcotics drive. "It appears to be genuinely more committed to drug eradication" than was Mexico's previous regime, he said.

The administration is not motivated by altruism, Mexican officials readily acknowledge, stressing their determination to prevent potential drug fiefdoms of the kind that dominate some South American provinces. "In Mexico, there is no force stronger than the state, and there is no zone where the state will permit drug traffickers to exercise control," Sam said.

Reflecting this attitude, the government has assigned 95 federal police to the investigation of the Camarena kidnaping, viewed as a direct challenge to its authority. During the past 10 years of a joint U.S.-Mexico program, no other U.S. agent is known to have been killed or captured by traffickers here.

Camarena's abductors are also believed to be responsible for the kidnaping later the same day in Guadalajara of a Mexican pilot who U.S. sources say flew missions for the U.S. Drug Enforcement Administration program there. They are not known to have communicated with U.S. or Mexican officials. "The longer the time that passes, the less chance there is of finding him," a U.S. diplomat said.

Concern about more attacks has prompted the U.S. Embassy to tighten "security measures, particularly for those individuals and officers and their families who might be targets," Ambassador John A. Gavin said yesterday. He added that U.S. authorities will be "removing from any area, and if necessary from the Republic of Mexico, the families of agents whom we consider to be in jeopardy."

The abduction of Camarena, who had operated in Guadalajara since 1980 as a liaison officer with Mexican agencies, is seen as a symptom of the rising stakes in Mexico's narcotics eradication campaign, a war some thought had long been won.

Guadalajara, Mexico's second largest city, was identified at a press conference here yesterday by Drug Enforcement Administration head Francis M. Mullen Jr. as "the major center" for several of the estimated "18 major gangs" he said are now active in drug trafficking in Mexico.

In 1975, when the DEA began its active role here, Mexico was estimated to provide 87 percent of the U.S. heroin supply and was just beginning to be edged out by Colombia as the principal U.S. marijuana source. But by 1981, Army-supported plantation raids and U.S.-financed herbicide spraying were said by a DEA official to have "dried up the illicit narcotics traffic" from Mexico. The country was thought to be responsible for as little as a fifth of the marijuana and heroin imported into the United States.

In the final months of 1984, however, Mexico was calculated to be providing 38 percent of that heroin, Mullen said. Even more alarming, Mullen reported, U.S. agents have also recently detected a near doubling of the purity of Mexican-produced heroin, once identified in the U.S. market as "Mexican brown" because of the taint of incomplete refinement.

Although Mexico is not a coca grower, U.S. analysts say as much as a third of the cocaine that enters the United States transits this country. Last year, Mexican police raided the first cocaine laboratory discovered here, sparking a crackdown on the sale of ether and other coca-processing materials.

In a November shoot-out that left one Mexican federal drug agent dead, officials confiscated 796 pounds of cocaine at a remote campsite 20 miles south of the U.S. border, an amount greater than all the cocaine seized in Mexico's previous history.

A week later, in the most spectacular indication to date of Mexico's narcotics revival, federal agents in the northern border state of Chihuahua incinerated an unprecedented 9,000 tons of mostly dried and baled marijuana. The amount equaled the estimated annual output of Colombia, thought to be the world's biggest producer.

Police also confiscated dozens of truck trailers and freight containers and arrested more than 11,000 marijuana pickers, packers, and warehouse workers. An exultant Jon Thomas, U.S. assistant secretary of state for international narcotics matters, called it "the bust of the century."

Organized three months beforehand, with DEA undercover work in the United States complementing Mexican surveillance of smuggling routes south of the border, the Chihuahua assault was "an exemplary example of international cooperation," Sam said in an interview.

The marijuana impounded in Chihuahua had been transported from every major Mexican growing region and from "at least one Central American country," Sam said, in an ambitious underworld attempt to "monopolize the marketing" of the region's crop. He said 140 traffickers and growers had organized the ring as "a kind of cooperative" and 26 of them are now in custody.

The November marijuana raid, postponed until the bulk of the harvest was warehoused at the clandestine distribution center, was also timed to take place after some U.S. customers had paid several million dollars for their share of the crop, sources said.

The Mexican coordinators of the drug ring had expected to collect an estimated $400 million for the marijuana delivered across the U.S. border, although its ultimate retail street value would have ascended to several billion dollars.

While some U.S. observers suggest that the Chihuahua raid dealt a crippling blow to Mexico's marijuana industry, Sam makes no such claims. "We don't really know what our national production is, so we don't know what percentage of it Chihuahua represented," he said. "We only know what we have confiscated or destroyed, not what we have missed."

Mexican marijuana growers "change their tactics almost daily," Sam added, sowing thousands of tiny, almost inaccessible and undetectable plots on remote mountainsides. Smugglers will probably revert to their earlier "decentralized distribution patterns," he predicted, making the prospect of another mammoth Chihuahua-style bust "very improbable."

Last year, aided by the Army, the Narcotics Control Directorate razed 9,400 acres of marijuana and 8,900 acres of opium poppies, more than double the 5,000 acres of marijuana and 3,150 acres of poppies destroyed by the government in 1982, the last year of Mexico's previous administration. "We have just flown more hours and gone more places than our predecessors did," Sam said.

Observers say this increase in drug destruction could reflect a rise in cultivation, brought on by two factors: heightened U.S. surveillance of Caribbean smuggling routes and the new cost-competiveness of Mexican land and labor, a consequence of 1982's drastic peso devaluations.

Sam said aerial surveillance is the most effective weapon in narcotics control, and the $123 million donated by the United States to Mexico's drug program over the past decade has been earmarked largely for the expansion and maintenance of the directorate's fleet of helicopters and spotter planes.

This year, the United States has allocated $7.9 million for aircraft purchases and repairs, equivalent to about a third of the directorate's budget. "No amount of money would be enough to do this job," Sam said.