SAN JOSE USUMACINTA, Mexico - Before dawn one damp October morning three years ago, Leonardo and Vicente Diaz descended a muddy slope deep in Mexico's southeastern rain forest, hopped into a canoe and crossed the big river the Aztecs called "place of monkeys."

The North American Free Trade Agreement had brought only pain to this corn-growing settlement. Young men like the two cousins, who dreamed of something better, were leaving. "If you stayed in the fields," said Vicente Diaz, "you would never get ahead in life."

Three bus trips, 35 hours and 640 miles after they set off, the Chol Indians, then 17 and 21, reached arid, mile-high Aguascalientes, where an uncle awaited them. Within a week they had jobs in the city's auto industry, which exports nearly 12,000 Nissan cars and trucks a year to the United States.

Their passage from a village with one motor vehicle and no toilets to a prosperous city with 11 industrial parks illustrates the trade-offs NAFTA has forced on Mexicans. In the treaty's first decade, more than 1 million of them have gained jobs manufacturing goods for export to the United States and Canada. But even more, 1.3 million, have been pushed off unprofitable farms by cheap American grain imports. And NAFTA has yet to improve overall wages, reduce the number of people living in poverty or close the gap between the rich and poor.

The drafters of NAFTA, which took effect in January 1994, envisioned a smoother adjustment for Mexico's 18 million rural poor. Places like San Jose Usumacinta would send laborers to export plants. Those staying behind would switch from corn to crops with a profitable niche in the North American market.

But with little credit or technical aid to make the switch, millions of farmers stuck with corn and sank deeper into poverty. And after 2000, growth of Mexico's assembly lines stalled because of the U.S. recession and the rise of low-wage competitors such as China.

The Diaz cousins are a NAFTA success story. But their success comes with the knowledge that they'll never return home -- and the burden of helping sustain their elders' farms from afar.

Each cousin earns $74.36 a week at Sanoh, a Japanese-owned auto parts supplier. That is an average wage for Mexico, but not enough, the cousins say, to let them help their families and build new futures for themselves.

"I send home what I can," Vicente Diaz said, turning down the boom box in his $55-a-month rented house in Aguascalientes. "The situation back there is depressing, something you would never want to live through. I left, but I have a permanent obligation to help them."

Leonardo's father, Anselmo, knows that his son is unlikely to return. "When the young leave here to follow their dreams," he said, "they do not come back."

The boom in Aguascalientes and the pain in San Jose Usumacinta grew out of policies Mexico embraced two decades ago and locked in place with NAFTA.

Corn prices tumbled in the 1980s as the government stopped guaranteeing purchases and phased out aid to the countryside. Although NAFTA set a 15-year schedule for eliminating tariffs on imported corn, Mexico was letting it in duty-free by the mid-1990s.

Midwestern U.S. corn growers, who can produce at least four times as much per acre as Mexican farmers, now meet nearly one-third of Mexico's needs. They have driven down corn prices by as much as 70%, according to groups representing the 3 million Mexican families who still grow the crop.

Mexican leaders, meanwhile, used tax incentives to lure foreign assembly plants, first to northern border cities, then farther south. Nissan, Texas Instruments and Xerox opened factories in Aguascalientes in the 1980s, positioning that city for a high-tech export surge under NAFTA. Nissan and a dozen Japanese auto parts makers now employ 13,683 workers in Aguascalientes, part of a burgeoning common market for auto production and sales that is one of the treaty's biggest achievements.

A well-managed city of blue skies, hot springs and 700,000 people, Aguascalientes has gained 62,000 manufacturing jobs under the trade pact and steady ratings as one of Mexico's best places to live and do business. Software engineers drive BMWs and Audis to attend the opera, play golf and shop at U.S.-style malls. Per capita income is $7,870, nearly $1,000 higher than it was 10 years ago and three times higher than in Chiapas state, where San Jose Usumacinta is located.

"There are two Mexicos -- one to the north of Mexico City, the other to the south -- and when you visit our city the breach between levels of well-being leaps out at you," said Ruben Camarillo, minister of economic development for Aguascalientes state. "NAFTA has not changed that."

To visit San Jose Usumacinta is to go back in time. One must walk five miles from the nearest drivable road in Chiapas or paddle across the Usumacinta River from Tabasco state.

Isolation defines the community, which was founded 40 years ago by Chol pioneers who hacked through the jungle. Villagers watched a single television set powered by a car battery until electricity arrived in 1989. Today, its 70 households own just six telephones and 12 refrigerators.

Like nearly half of Mexico's population, all 415 villagers live below the official $4-per-day poverty line. Men plant corn by poking the soft earth with sticks. Women walk all day to collect bimonthly welfare payments of as little as $28 per family. A midwife delivers the babies, who grow up suffering diarrhea contracted from tainted well water.

"The first time I visited, I got goose bumps," said Eric Suriano, who doles out the welfare from a federal anti-poverty program. "I did not realize human beings still live that way."