Our troubled Mexican neighbors have four major lines of export to the United States, and two of them are outright illegal. Out in the open is the shipments of oil and farm products that reach U.S. markets every year.

Illegal but understandable is the northward surge of hundreds of thousands of poverty-impelled Mexicans who sneak into the United States, desperate for underpaid jobs on our ranches and in our cities.

But the most lucrative export is the most sinister. It is the stream of "Mexican mud," or heroin, that originates in the poppy fields of the Sierra Madre and ends up in U.S. drug traffic. The heroin trade across the border enriches the Mexican suppliers by an estimated $500 million a year and multiplies in value when it hits the illegal U.S. market. Shadowy organizers of smuggling rings have become millionaires on both sides of the Rio Grande. And it has left a backwash of crime corruption and ruinous addiction in both countries.

These disparate economic worlds come into focus in the fertile Mexican state of Sinaloa, and its capital city of Culiacan. Sinaloa, known as the "bread basket" of Mexico, harvests over half of the country's total agricultural production. Over 80 per cent of Mexico's heroin-poppy growth also comes from the isolated Sierra Madre range in Eastern Sinaloa.

Culiacan is both the headquarters of the profitable winter vegetable business and a key refining point for Mexican heroin. And in the crowded barrios of the city and the surrounding rural shantytowns are found the impoverished factory and farm workers. Some drift north to the United States as illegal aiens to seek better pay in the California fields.

We sent out associate Hal Bernton to Sinaloa to investigate the connections between vegetable growers and heroin traffickers. He learned of absentee plantation owners and newly rich drug millionaires living check by jowl in plush villas on the tree-lined streets in Culiacan. In some instances, wealthy growers reportedly are amassing hidden frotunes from the heroin trade.

U.S. police sources report that some vegetable producers are deeply involved in drug operations. Our reporter double-checked these reports with U.S. narcotics officials who have worked in Mexico. Independently, Jacques Kiere, the top U.S. drug official in Mexico City, told Bernton he knew of "a few reports of heroin . . . discovered being smuggled underneath the vegetable produce in trucks." He stopped short, however, o f directly involving major growers.

Other officials, while declining to name names, had heard of participation in the heroin operations. These statements were backed up by sources inside Mexico.

Mexico's inner ring of heroin traffickers is difficult for police agents to penetrate. Most of the bigshots have insulated themselves from direct involvement by layers of underlings. Major dealers also have strong political ties with local government officials, who hamstring police efforts.

An incredible cash flow from heroin into Culiacan has made it one of the most prosperous cities south of the border. Expensive U.S. luxury items disappear quickly from the shelves of stores in the central city. Cadillacs and Ford LTDs are best sellers in auto showrooms.

But the coming of heroin prosperity has exacted a fearful toll among Culiacan's law-abiding inhabitants. The town took on the semblance of Chicago in the days of AI Capone as warring gangs of drug dealers terrorized the citizenry. At nightfall the residents abandoned the streets and huddled behind locked doors. In the dark, drug gangs, toting machine guns and automatic rifles, emerged to engage in deadly shootouts.

In 1976, Culiacan's murder rate ranked among the highest of any city in the world. Policemen who dared to challenge a gang's sway risked assassination.

The Tierra Blanca section on the eastern side of the city became a no-man's-land to outsiders until last year. Then the Mexican government launched a crackdown and sent in army troops. They virtually occupied the city in support of the beleaguered Culiacan police.

Today there are resurgent signs of normalcy. Families stroll the streets, and young couples embrace in the town park. Mime shows play to evening crowds that gather in the central square.

But pickup trucks full of heavily armed soldiers constantly patrol in Culiacan. This near state of siege is a reminder that the gangs may be quiescent but still remain in the shadows. The calm is an uneasy one, and a reduced flow of Mexican mud to the North goes on.

Footnote: The U.S. - Mexican effort to destroy the poppy fields in the high Sierras has dealth Culiacan's heroin economy a severe blow. Helicopters have searched out the poppy fields in the crags and corners of the rugged mountains and destroyed them with poison sprays. The Environmental Protection Agency now wants either to ban or highly restrict one of these sprays in the United States. They believe that the paraquat spray is hazardous to wildlife and may cause pulmonary fibrosis in humans.