The nameless young man darts through the rush-hour traffic, then bounds up two flights of stairs in an abandoned building frequented by junkies.
In minutes, in the flurry of the Dutch capital, he has bought enough heroin for another night, ''fixing'' his habit until tomorrow.
Some 200,000 heroin addicts in cities across Western Europe suffer in the same way - an alarming number considering the problem barely existed a decade ago.
Now, following a bumper crop of opium harvested from poppies recently in Southeast Asia, the problem could grow even worse this year, according to authorities. The opium is refined into heroin for use around the world.
''We're beginning to be inundated with 'golden triangle' produce,'' says an agent with the United States Drug Enforcement Administration (DEA). He works with other Europe-based American narcotics agents in cooperation with local authorities to track traffickers and seize shipments.
In recent weeks police in the Netherlands - the point of entry for much of the heroin destined for the wider European market - have seized several ''significant'' shipments (i.e., packages weighing up to 10 kilograms each) coming from the opium-rich ''golden triangle'' (the region where borders of Thailand, Burma, and Laos intersect). Dutch police intercepted a total of 60 kilos (or 132 pounds - worth about $2.5 million on the European street) in the last quarter of 1981, a sharp rise over ''normal'' levels.
Besides boding well for traders and ill for drug enforcement officials, the increasing influx of ''golden triangle'' heroin into Western Europe could signal a shift in world trade and production patterns, some officials point out.
Until last August, shipments from Southeast Asia had fallen to a trickle. Several large trafficking syndicates had been dismantled. Opium production was low.
''Now production is blossoming,'' says one DEA official. ''New organizations have grown up, and not surprisingly we're beginning to see the results of that changing situation in Europe.''
Drug-enforcement agents, who have tightened surveillance at entry points in Western Europe, consider their job difficult.
''Most every ship calling at Rotterdam, for example, is capable of carrying one to two hundred tons of heroin,'' says one DEA official. More than 300,000 ships call there every year. This means that unless we know what ship it's on, it's virtually impossible to stop the stuff from entering Europe.''
Many officials feel that the best hope for halting the flow of heroin from faraway places is therefore to hit hard and destroy the source.
''But it's not easy,'' one official says. ''Destroy one group, and another hundred are ready to step in to fill their shoes.''
In Southeast Asia there has been cautious optimism following just such an effort to strike at one major source. Recent raids by Thai soldiers on operations of ''golden triangle'' opium warlord Khun Sa may significantly cut into this year's heroin production in Thailand, according to foreign narcotics agents based in Bangkok.
But large amounts of opium continue to be grown in Burma and Laos. And unless the drive against narcotics is continued in Thailand, the raids will probably only disrupt the flow of heroin temporarily, a narcotics official here cautions.
Drug enforcement officials fear that increasing ''golden triangle'' production could have a far more damaging effect on Western Europe than on other countries, because the low grade of heroin commonly manufactured there is acceptable to users in Europe - particularly among the ethnic minorities.
The influx could not have come at a worse time. Drug abuse in Western Europe has grown by leaps and bounds in recent years, reaching critical proportions today.
At a meeting last year to consider the growing problem of drug abuse in Europe, ministers from the 21-nation Council of Europe - an advisory body for European governments - expressed shock at the increase of drug abuse and noted the spread of the habit to ''new sectors'' of the population.
''The proportion of women addicts, for instance, has increased considerably, '' the ministers said in a statement. They heard reports that the number of heroin addicts in the 10-nation European Community had risen to about 200,000 (compared with about 600,000 in the US), about 15,000 in Britain, 20,000 in France, and 50,000 in West Germany.