One post-World War II President after another had found it politically advantageous to exploit, or at least cater to, the American public's near-hysteria over narcotics and drugs. Jimmy Carter, to his credit, is an exception.

Indeed, in his five year in office, he has made a constructive start on putting the problem in saner perspective, so that, one hopes, it can be dealt with more rationally and effectively than has been the case in recent years.

What a contrast his policy is with the cynical way former President Nixon whipped up public fears in his first campaign for the White House. Then, after getting there, he staged his so-called "international war" on drugs, which was good for alarming headlines, but little else.

As part of his campaign, Nixon prodded the Mexican government into jailing hundreds of American youth allegedly engages in narcotics trafficking. Minor first offenders often got the kind of harsh sentences ordinarily meted out for capital crimes.

So many horror stories of torture to these young inmates leaked out that the Mexican government, fed up with the damaging publicity, this year agreed to an exchange of prisoners with the United States, which Carter has put into effect.

Recently, the front pages have been filled with pictures of the returning youths. That they were welcomed back more like martyrs than criminals indicates how this emotional issue is now gradually being defused by the Carter administration.

One of the first things Carter did after taking office was to establish a White House Office of Drug Abuse Policy and install a trusted associate, Dr. Peter Bourne, as its director. Bourne, an expert in the drug field, has approached the problem broadly and dispassionately.

In the spring, when the administration endorsed decriminalizing the possession of marijuana, Bourne told the House Select Committee on Narcotics Abuse that present criminal sanctions against marijuana possession were "counterproductive."

The congressmen were also impressed by his documented testimony that, when otherwise law-abiding young people go to prison, it "causes far greater harm to their lives than any effect the drug would have had."

Bourne said 35 million Americans have tried "pot," and as many as 11 million are now using the drug at least weekly. His data was backed up by the National Institute of Drug Abuse, whose director says, "There is no question that alcohol and tobacco are causing us far more health problems than marijuana does."

Despite the administration's enlightened stand, a still timid Congress has not yet acted on any of the decriminalization bills before it, but sentiment is definitely moving in that direction. Also, several additional states, notably New York and Mississippi, have decriminalized it this year along lines approved by Bourne, and it is a good bet that half or more of all the states will do likewise in the next year or two.

Meanwhile, federal agents and prosecutors are taking their cue from the administration, with the result that few citizens are now being arrested or prosecuted for violating national laws against the mere possession of small amounts of marijuana. Many local and state law-enforcement officials are following suit.

Bourne has also been encouraging a more realistic approach to the harder drugs, such as heroin. "All drugs," he recently said, "should be examined purely on their medical and scientific merits. And that decision on making them available to patients should not be based on the drugs' history for abuse or their reputation in society."

Even before going to the White House, Bourne felt the time had come to re-examine heroin and set up treatment centers to dispense it legally. It's a matter of "exploring, not advocating." he cautions.

He was speaking for himself, rather than the administration, but the idea is rapidly spreading. The National League of Cities, for instance, has been debating a recommendation to study "the feasibility of a heroin-maintenance strategy."

The Massachusetts Council of Churches has also urged national consideration of carefully supervised heroin clinics for confirmed addicts. The council believes the choice is "whether we want heroin procured on the streets or through health clinics."

Perhaps the most surprising initiative sprang from a group of old-fashioned citizens banded together as a grand jury in San Siego, Calif., and headed by a foreman who is a retired Marine Corps general.

Concerned over reports that the area's 3,000 addicts get $500,000 a week through thefts and holdups to maintain their habit, the grand jurors, taking a cue from Britain, proposed the distribution of free or low-cost heroin by local clinics "to certified addicts with no attempt at rehabilitation."

Nobody knows for sure how many addicts there are in the United States, but the accepted estimate is 500,000, as compared with only 3,000 in Britain, where registered addicts can get heroin legally for 25 cents a shot (or free) instead of $50 to $100 a "fix from pushers, as is the case in America.