Critics charge US is doing too little to halt drug trade at source. Experts say the US war on drugs must include cutting off supply in addition to easing demand. Has the US been tough enough, for instance, in slashing coca production in the Andes?

On paper it seems such an easy solution to America's growing cocaine problem. Simply convince peasant farmers in Bolivia and Peru to grow something other than coca (the raw material of cocaine), and the world's largest source of cocaine would begin to dry up.

But the reality on the steep slopes of the Andes is that, pound for pound, illicit coca production is by far the most profitable enterprise available to peasant farmers. In the view of many rural Bolivians and Peruvians, it is the best thing that has ever happened to them.

But is the United States doing all it can to reverse that perception among South American farmers?

Some critics say more can, and should, be done to attack the cocaine problem at the source, while the future narcotics are firmly rooted in one place and growing in the open sunshine. They point to the mountain slopes of Peru and Bolivia, where more than 90 percent of the cocaine smuggled into the US is grown.

In coming weeks, the Reagan administration and Congress will be conducting a major reassessment of America's war on drugs.

That effort is expected to focus attention on the so-called demand side of the problem -- American addicts and drug users. But it will also address whether the administration is doing enough in battling international narcotics traffickers and growers.

In recent years, the US government has spent more than $8 million a year in Peru and $7 million a year in Bolivia on efforts designed to bring illicit coca production under control. During the same period, coca production has doubled in Peru and quadrupled in Bolivia, according to conservative estimates.

Meanwhile, cocaine shipments to the US are at an all-time high, as are US statistics recording cocaine users, cocaine overdoses, and cocaine-related deaths. And in major cities across the country, law-enforcement officials are struggling with the newest manifestation of the cocaine glut -- a cheap and highly addictive crystalline version known as ``crack.''

``We are not making any headway, we have not even halted our losses,'' says John T. Cusack, chief of staff of the House Select Committee on Narcotics Abuse and Control. ``[The drug traffickers] have spread cocaine all over this country. ``There is hardly any place you can go in the United States where you can't find cocaine,'' he says.

Mr. Cusack says the Reagan administration should be more forceful in insisting that Bolivia and Peru stamp out illicit coca production in their countries. If necessary, he says, the US should withhold all aid until both countries undertake an effective crackdown. Cusack says US policy has been one of ``buying time'' rather than taking swift, results-oriented action.

In the meantime, the proliferation of cocaine in the US continues.

State Department officials respond that their critics do not understand the complexities of Bolivia and Peru. ``Most people in the US fail to recognize how formidable this problem is,'' a State Department official says.

US officials say they are beginning to see the fruit of years of careful maneuvering by diplomats and drug-enforcement experts. The recent crackdown in Bolivia against cocaine laboratories, with the help of US military helicopters and pilots, is being hailed as the first sustained commitment by the Bolivian government to bring coca production under control.

Drug-enforcement efforts in Peru and Bolivia have traditionally been complicated by the logistics of rooting out coca bushes there. The crop is grown on remote and forested mountain slopes where it is difficult to detect, and where government authority is virtually nonexistent and the influence of the drug traffickers is growing.

To move too quickly in eliminating coca plants might spark a rural uprising among peasants who depend on coca for cash or who are in league with the traffickers, officials say.

Terrorist and other violent incidents were directed against government officials working to replace coca production in Peru in 1984 and '85. And Bolivia in recent years has experienced a string of government coups amid the rise in power of narcotics kingpins.

``It was only a matter of time before the narco-industry took control of the country,'' says one State Department official, referring to the threat in Bolivia.

The official added, ``It is important that, as the democratic process moves forward [with the new Bolivian government], the United States functions in a way that strengthens that democractic leadership. And you don't strengthen them by strong-arming them.''

Cusack says he is aware of the complexity of the problems in Peru and Bolivia. He served as chief of overseas operations at the US Drug Enforcement Administration in the 1970s, when the Turkish government successfully stamped out that country's opium trade. Turkey had been a prime source of heroin in the US before the crackdown.

``This is not really [a matter of] the poor farmer growing a little coca on the side to help out his family,'' Cusack says. ``This is organized and subsidized by landed people, professional people, and warlord-types of characters.''

Cusack stresses, ``The government has to tell them `No.' ''

Current US strategy in Peru and Bolivia is to wean peasant farmers from reliance on illicit coca. The US is funding broad development projects in rural coca-growing regions designed to help replace coca production with alternative crops and create other job opportunities for growers.

At the same time, US officials are urging wider police actions in Peru and Bolivia to make it more difficult and costly for the traffickers to operate. It is hoped that the combination of stepped-up enforcement and increased economic opportunities will spark farmers to decide against growing coca.

But Cusack and other experts question the effectiveness of this strategy, noting that similar development projects in Thailand and Pakistan have had only mixed results. They say that unless crackdowns are extended nationwide, farmers in Peru or Bolivia will continue to increase their coca production to compensate for coca bushes destroyed in US-backed eradication programs. ``The first thing we have to do is stop the constant increase in [coca] production,'' says Cusack, who stresses, ``We haven't done that yet.''

``There is crop substitution going on [in Bolivia and Peru], but they are substituting coca for potatoes, coca for corn,'' Cusack notes. ``It is going the wrong way.''

State Department officials admit that progress has been slow, but they emphasize that they are laying the groundwork for future solutions. They note that in the aftermath of the Bolivian government's recent anticocaine raids, the price of 100 pounds of coca leaf has fallen from $100 to $20. The cost of growing 100 pounds is estimated by officials at $40. Some Bolivian farmers have already begun to ask about alternative cash crops, officials say.