ADERA, Calif., May 12 — Along the country roads off Highway 99, it is plain to see why the Central Valley calls itself the nation's fruit basket. Rising from some of the richest soil in the world, disciplined rows of fig and almond trees give way to orange and lemon groves, cherry orchards and bushy lettuce and cabbage plants, as far as the eye can see.

But hidden away on this soil, in abandoned barns and falling-down farmhouses, hundreds, if not thousands, of laboratories are churning out illegal methamphetamine, the highly addictive stimulant that Barry R. McCaffrey, the former federal drug czar, has called "the worst drug that has ever hit America."

As a result, methamphetamine is likely to be one of the biggest challenges for President Bush's newly nominated drug czar, John P. Walters, and the man Mr. Bush selected to run the Drug Enforcement Administration, Representative Asa Hutchinson of Arkansas.

In the last few years, the Central Valley, particularly its heartland, has become so inundated with methamphetamine laboratories — many of them run by Mexican crime families — that the Drug Enforcement Administration has labeled it a "source nation" for the drug. The valley's only competition, federal authorities say, is Southeast Asia, which produces and distributes the drug in pill form, mainly to Europe. Here the drug is produced as a powder, which users snort, inject or even slip into their coffee.

"It's been growing tremendously in the last five or six years," said Joe Keefe, chief of operations at the drug agency. "In 1996, we looked at methamphetamine trafficking by the Mexican nationals and had 60 investigations. In the last couple of months, we had over 800." The organizations have also expanded their marketing all over the country, he said, such that methamphetamine produced in California can be bought on the street in Portland, Me.

Other states, particularly Washington, Missouri and Iowa, also have significant problems with methamphetamine laboratories, but 97 percent of the "superlabs" that can be traced to Mexican drug operations are in California, law enforcement officials say. The state produces 80 percent of the drug found in this country, the officials say, 60 percent of it in the pastoral towns of the Central Valley stretching from Bakersfield to Sacramento.

Government officials consider methamphetamine the fastest-growing illegal drug in this country, in Canada and in parts of Europe, feeding an epidemic of addiction that they say rivals that of heroin and cocaine over the past few decades.

But the impact is felt acutely here as the clandestine laboratories poison the Central Valley's soil with byproducts and tax the combined resources of special squads from dozens of law enforcement agencies. Officials have also expressed particular concerns about children who live in or near the laboratories and are exposed to dangerous fumes.

In the last decade, officials say, methamphetamine production has surged in the state as a whole and in the Central Valley in particular. In 1999, 261 laboratories were seized in 9 of the valley's 17 counties, triple the 73 seized seven years before.

But the cartels, officials say, see the raids simply as the price of business. When a laboratory is raided or found accidentally — sometimes when the cooks blow up the building they are in — the operation simply finds another barn or house.

This makes the operations particularly hard to break, said William Ruzzamenti, a special agent for the Drug Enforcement Administration and director of the Central Valley High Intensity Drug-Trafficking Area program. The Central Valley program, which began in January 2000, operates four task forces from more than 50 federal, state and local law enforcement agencies that comb the valley for the laboratories.

They are relentlessly busy. Central Valley's methamphetamine task forces and other law enforcement agencies crack five laboratories a day in California. The amount they seize is only about a tenth of the methamphetamine produced, officials estimate.

The drug cartels out-finance the antidrug efforts many times over. The Central Valley task forces, for instance, receive $2.5 million a year in federal aid to fight the producers.

"We keep busting them," Mr. Ruzzamenti said. "But they keep setting up shop."

Methamphetamine, widely known as meth, crank and crystal, was once produced and sold solely by outlaw motorcycle gangs, drug officials say. In the 1960's and 70's, the gangs cooked the product in remote outposts in the California desert and distributed it themselves. Then, in the early 90's, as crack waned, Mexican crime families, primarily from Michoacán, who had been trafficking in cocaine from Colombia, discovered that they could make more money by creating their own product, which they would not have to smuggle to the United States.

In places like San Diego, San Bernardino, Riverside and Los Angeles Counties, they began setting up the superlabs — those that produce at least 10 pounds a day, unlike the smaller, amateur laboratories run by drug users.

But aggressive law enforcement efforts began putting a crimp in the superlabs, and about four years ago, officials say, the cartels began moving operations north to the San Joaquin Valley, the wide-open section of the Central Valley.

Law enforcement officials say that shaking the superlab operations is particularly hard in the Central Valley because its vast, unpopulated stretches and ready access to interstate roads make it easy to hide and transport methamphetamine. Also, the valley's chronic high-unemployment rate makes recruiting workers, ignorant of the deadly risks of producing the drug, as easy as selling lemonade on a hot day.

"One of the tragedies of this business is that the crime families consider the work force a renewable resource," Mr. Ruzzamenti said. "When the workers get too sick from all the chemicals they've been ingesting to keep going, they just bring over or recruit others."

Any unassuming building can be a methamphetamine laboratory producing up to 100 pounds per 24-hour cooking cycle. Robert Pennal, commander of the Fresno Anti-Meth Task Force — which covers three of the most active counties, Madera, Fresno and Merced — has learned to look at every building in the middle of a field with a suspicious eye.