ONE OF THE BEST ways to meet the people of Jamaica these days is on a Honda 50 motorbike. As you cruise through the mountainous interior, men, women and children walking along the highway flag you down for a ride and quickly strike up a conversation.

"How long you been here, mon?"

"Oh, coupla days."

"You want some ganja, mon?"

Ganja is the ubiquitous, powerful Jamaican marijuana, a cash crop worth an estimated $3.5 billion a year, a sum larger than the country's official gross national product. And to both the casual visitor and U.S. congressional investigators, it is literally everywhere.

Take a typical day on the beach at Negril, a seven-mile stretch of resorts on white coral sand and clear blue water at the western tip of the island.

You check into your hotel, slip on your swim trunks and walk the 30 yards to the beach. You lay down your towel, but before you can make it another five paces to the water, you meet your first "greeter," a friendly Jamaican man usually under 30 who says something like, "How long you been in Jamaica, mon?" And so on.

The pitches for a ganja sale can recur to a level of annoyance, as a constant stream of peddlers selling wood carvings, fresh orange juice, baskets, bananas or coral bracelets parades by your towel.

You don't want ganja? How about the locally manufactured hashish, psychedelic mushrooms or cocaine (which a couple of peddlers told me is now being imported to Jamaica from the United States)? The pitch is persistent.

All of which has confronted Edward Seaga, the conservative prime minister and Ronald Reagan's closest friend in the Caribbean, with a stiff political and economic challenge. As his island's earnings fall with the dwindling price of sugar and bauxite, and with perhaps half of its workforce officially unemployed, citizens are ever more dependent on harvesting dope for export or sale to tourists.

Traveling around Jamaica I found whole villages which had made the transition from dependence on legal commodities to a vast and lawless economy based on marijuana. One might say that the Jamaican marijuana production is a shining example of the free market at work -- the very kind that the Reagan administration extols. The irony, of course, is that the administration has made the war against drugs a keystone of its foreign policy toward Jamaica, promising economic aid in exchange for action against the drug trade. In effect, the question is how to get Jamaicans back to sugar cane or other sources of income after they've seen the kind of cash that dope can generate.

Nowhere is the dilemma more graphically evident than in Maggoty, a village I encountered high in Jamaica's west-central mountain region.

Maggoty seems drawn from a V.S. Naipul novel, situated as it is on a bend in the river. It is a small port of colonial-era, one-and two-story buildings on the navigable Black River, which is said to be infested with a local variety of crocodile. I approached it from inland, meandering through a vast, humid valley of sugar cane belonging to the famous Appleton rum and molasses company. As I drove along the bumpy deserted road, I suddenly came upon a stark rusting hulk of an abandoned minerals refinery, near which was the beginning of a slurry pipeline big enough for a man to walk in, and which snakes across the river and down through a valley of impenetrable underbrush and thick forest to Jamaica's southern coast some 25 miles away.

Gliding at last to a stop in Maggoty on a scorching-hot October noon, I came upon a few elderly men loitering on the front stoop of a closed general store in the town's dusty main square.

After asking directions, I fell into conversation with one of the men, who told me I had passed a bauxite refinery that had shut down around 1976.

"And what did the people do after that?"

The man glanced down at the crumbling asphalt and kicked a pebble. "They all went farming," he said.

"For the ganja?" I asked.

He looked up at me and nodded yes. We stood together in a moment of unfocused embarrassment.

"The entire town?"

"Yah, mon. De're is nuh-ting else here, mon, but de ganja. Nuh-ting."

There wasn't much else to talk about at this point, so I cranked up the Honda and asked about the road ahead. With firm warning, he told me to take the longer route around the mountains, and not to go up further into the hills. The clear implication was that the other route would have taken me into hostile "ganja country."

Only after I returned to the United States and examined government maps and documents did I discover that I had unintentionally spent three days traveling on my motorcycle alone through the heart of Jamaica's marijuana-growing region. It is territory where the crop is tended so carefully that it "looks like a Japanese rice paddy," in the words of an awed congressional drug investigator who flew over the island last year.

Photos taken by the House Select Committee on Narcotics Abuse and Control confirm it: The marijuana plants -- acres upon vast acres of them -- are tended and pruned in long irrigated rows not unlike Florida citrus groves.

The impression grew as I traveled the back roads of Jamaica that I was witnessing the evolution of an entire country toward dependence on marijuana -- the creation of a "Ganja Nation," an exponentially inflated version of what had happened in some small fishing towns in Florida when ordinary citizens grew weary of unemployment and became rich drug smugglers.

In town after friendly town where I stopped, Jamaicans told of ganja farming replacing sugar, bauxite and, in one case, indigo dye, as the source of money and jobs. And one depressing corollary emerged from my conversations, especially with young people: the ganja trade was the one, foolproof means to make enough money to get off the island and secure some kind of future -- hopefully in America.

The island is laced with "clandestine" airstrips. A recent edition of Jamaica's Daily Gleaner newspaper featured front page aerial photos of several, accompanying a government announcement of an imminent crackdown. Such strips are ever more ambitious. One that I accidentally came upon in the wilds of the southwest coastal area could have serviced Vietnam- era cargo planes.

Photos taken by U.S. investigators show one strip with a twin-engine transport plane parked on the dirt runway. For years, Jamaican experts say, an empty spot on a back road would have been perfectly adequate for a quick loading of a single-engine Cessna. For bigger hauls, local dealers point out, the profits are so immense that it's worth the effort to bulldoze a strip for a one-time use.

Ronald Reagan has extracted a promise from Edward Seaga (his first official visitor at the White House in 1981) to eliminate the export of marijuana. Over $5 million in military aid has been promised for 1985, and an increasing portion of nearly $80 million in economic and development aid is being allocated to anti-drug programs. Naval, air and radar surveillance has been increased, and the Jamaican police have allegedly placed informants on resort beaches. Sources say the U. S. Drug Enforcement Administration will be increasing its assistance to Jamaica this year. Plans even include the development of anti-ganja paramilitary raiding teams.

To the casual visitor -- and to congressmen who visited last year and expressed their concern -- little seems to have been accomplished, except for a dramatic increase in drug-related violence (16 police have been murdered in the first nine months of this year), high-level corruption and some hardship on small-time dealers.

As to the latter development, one hapless ganja peddler told me an illustrative story:

"Lionel," we'll call him, is a part-time taxi driver who shared a dinner of scrawny baked chicken, rice and beans with me in an otherwise empty Negril cafe one night. He said he used to work as handyman on the estate of a moderately successful dealer who exported dope off the island in small planes. There was plenty of work, and regular pay. Then a Jamaican police raid shut down the operation, and Lionel lost his job. Now, to make ends meet in the off-season, he maintains a small plot of ganja in the hills nearby and hawks it to tourists. "Everybody got to eat, mon," he says.

Jamaican drugs are not "everywhere" in the sense of, say, Adams Morgan, where a resourceful pot smoker can find a "nickel bag" of grass with little effort. So many of the Jamaicans a tourist meets -- cab drivers, waiters and, of course, the ubiquitous hitchhikers -- hawk drugs that there is a gas station-style "ganja war." In the glutted ganja market, service with a smile and all the other marks of successful salesmanship can make the difference between eating rice and beans and driving a cab or your own new Toyota.

It is a milieu suited to the budding entrepeneur, and like any promising growth business, it has attracted foreigners with the time, money or imagination to make or at least contemplate their own fortunes. Typical of those who are joining the ganja rush was the 21-year-old German girl from Berlin's Free University whom I encountered on the beach one day. Fifteen years ago, she would have been called a hippie, entranced, as she was, by the seemingly exuberant and friendly Jamaican culture, especially its sing- song patois and, of course, the ganja. "It's really cool, mon," she liked to coo into a bloody-red, ganja-swollen Jamaican sunset.

As this incipient capitalist saw it, there was a huge market for the famed Jamaican dope in West Berlin, and she had been researching the possibilities carefully. Eager local suppliers, she indicated, had been falling at her feet, and there was a sizeable community of exile Jamaicans in West Germany to distribute the drug once it got there. According to her market studies, transport by ocean-going yachts would be most reliable.

The Reagan administration's apparent enthusiasm for building paramilitary muscle and spy networks in Jamaica is unlikely to interfere with the dynamic ganja trade, which is responding to the laws of supply and demand.

For one thing, according to a congressional aide, there is not much police and military infrastructure to build on. "It's somewhat frustrating in one sense, because of the fact that the defense establishment down there is very thin," he said. "So in one sense we're talking about building up a defense and police structure. That's the first thing that needs to be done -- train and equip an entire infrastructure there."

Ganja farmers, however, have found ways to respond to even the slightest pressure; according to the House committee's report, they have been known to burn down their neighbors' sugar cane crops, resulting in local pressure against the government to lay off. Tougher police measures, experts warn, could lead to Bolivian-style "narco-terrorism," in which dope lords engage in kidnapping, murder and extortion, and whole sections of the country slip out of government control.

Already the Jamaican capital of Kingston resembles a drug-wars version of Beirut, with rival gangs controlling swatches of the slums and shooting it out daily with each other and the police.

The U.S. administration is promoting the "hardware" approach to eliminating Jamaica's booming marijuana trade, emphasizing interdiction of the export traffic with high-speed patrol boats, helicopters and radar-laden surveillance planes and modern police equipment for the Jamaicans. This pleases law enforcement bureaucracies in both Washington and Kingston by expanding state-of-the-art equipment inventories, personnel, and budgets. At the same time, the approach neatly sidesteps the thornier economic and political problems of eradicating ganja where it is grown.

Jamaica did flirt with eradicating ganja at its source a decade ago. In 1974-75, under Prime Minister Michael Manley, the government launched "Operation Buccaneer," an ambitious effort to destroy marijuana acreage and shut down clandestine airstrips. According to the House report, "this dramatically curtailed the production and traffic of marijuana for sevral years." By the late 1970s, however, the Manley government was in deep political and economic trouble, and the drug program was dropped. By 1980, production returned to previous levels.

As in most attempts to estimate drug production and trade, the figures on Jamaica are based on guesswork. According to Drug Enforcement Administration spokesman Con Dougherty, 1983 Jamaican marijuana exports to the United States amounted to about 1,750 metric tons, or 14 percent of the American market. At wholesale prices, the value of that amount of dope would be $2.3 billion. Even if the export figure is inflated, it gives an idea of Jamaica's enormous dependence on the crop.

The Reagan administration has pledged to make Jamaica a showcase for its Caribbean Basin Initiative economic development plan. For his part, Prime Minister Seaga, elected in 1980, has pledged to Reagan that he will curtail, if not eliminate, the ganja trade. Surely, however, he understands the pitfalls in actually doing so. Thus, the attraction of the "hardware" approach to interdicting the drug trade: It doesn't require the economic and political dislocation that would come from trying to substitute other crops for ganja.

In theory, crop substitution looks promising as a way out of Jamaica's economic dilemma. "They (the Jamaicans) should start bing down all the sugar cane fields, except for those needed for the rum industry," says a foreign government official who spent five years on the island in the late 1970s. "There's too much of it in the world already." Then, he says, the government should undertake a massive effort to grow more export crops, such as citrus, banana, and avacado, and raise more beef. But that probably would require a volatile transition period, one which Seaga would more happily leave to his successors, as he simply lacks the political base to launch such radical policies.

In the absence of serious attempts at crop substitution, the House delegation that visited Jamaica in 1983 heard Seaga gamely boast of increasing interdiction efforts, but noted in its report that "this, in effect, is a restatement of the strategy followed by the government of Jamaica, with the exception of the 'Operation Buccaneer' period, with no success since the late 1960s." Rep. Charles Rangel (D-N.Y.), chairman of the Select Committee on Narcotic Abuse and Control, lectured Seaga that U.S. economic assistance to Jamaica would be threatened unless a more direct strategy were undertaken. But the Reagan administration has almost doubled aid since 1981.

Governing a country that floats precariously on the rim of a deep vortex of poverty common to Third World nations dependent on a few commodity prices, Seaga would have to undertake heroic measures indeed to attack the source of ganja revenues. Except for a promising tourist industry, no other significant sources of revenue have appeared on the horizon. Meanwhile, Jamaica is under great pressure to devalue its already-wilting currency, and the World Bank and International Monetary Fund are both pushing for further cuts in social services to free up funds to pay Jamaica's foreign debt.

So for the Jamaican economic future, it is likely to be rum, ganja and rebellion against drug laws, mixed with hopes that tourism can take up some of the slack in currency earnings.

Americans can debate the merits of traditional foreign aid all they want, but the lesson of Jamaica is that, in the bigger picture, the money doesn't add up to that much. The fact is that the United States' huge imports of dope are people-to-people assistance of the most direct kind. In Jamaica, Colombia, Bolivia, Peru -- not to mention the drug-exporting countries of the Middle East and Southeast Asia -- thousands if not millions of farmers, rural people, and young entrepreneurs are getting income from the drug business.

This may be a major headache for U.S. law enforcement agencies, but it is also one of the biggest transfers of U.S. capital to the Third World today. That, of course, is not something the U.S. government can boast about in its economic dialogue with developing countries. Nor is it something for which the Jamaican government can publicly express its gratitude.