For most of the postwar period, Latin America was virtually off-limits to U.S. merchants of advanced supersonic aircraft.

General Dynamics, McDonnell Douglas, Northrop and the big engine manufacturers that supply them had to stand by while France and the Soviet Union, exploiting U.S. government restrictions on the sale of advanced aircraft to the region, carved up the market for themselves.

That is changing under the Reagan administration, which has specifically rejected the Carter administration's controversial policy of "restraint" in U.S. arms sales and has served notice that "the United States will not jeopardize its own security needs through a program of unilateral restraint."

The administration has approved the sale of General Dynamics' F16 aircraft to Venezuela.

In March, 1981, it authorized Israel to sell Ecuador 12 Israeli-made Kfir fighters equipped with U.S.-made General Electric engines.

More recently it has permitted Peru to start acquiring technical information from General Dynamics about F16 aircraft outfitted with a slightly less souped-up engine than the one used by the U.S. Air Force.

These deals and overtures are only a few of the more dramatic signs of a new American arms sales policy under which the United States is now committing itself to sell sophisticated arms in regions and to many countries to which it was previously thought unsafe to sell, and to which such weapons were denied previously.

For fiscal 1983 the administration has asked for $1.8 billion more in military aid, a 40 percent increase. This money would be used for such purposes as supplying F16s to Pakistan, which has been denied new "lethal" U.S. weapons systems since its 1965 war with India, and laser-guided bombs to Oman, a desert sultanate inhabited by 930,000 people.

The administration has notified Congress that it plans to sell improved Hawk missiles to the United Arab Emirates, Maverick missiles to Morocco and AN/TPQ mortar-locating radars to Singapore and Thailand. India reportedly is interested in F5G fighter aircraft.

U.S. arms control officials and private experts say this new open-door policy to arms buyers is dictated only in part by the philosophical inclination of the Reagan administration to arm countries seen as threatened by U.S. enemies such as the Soviet Union, Cuba or Libya.

They suggest that the change of heart already was beginning to take shape in the latter part of the Carter administration, when the Soviet invasion of Afghanistan and U.S. impotence in Iran caused a reassessment of "restraint" and a search for new ways to project U.S. power, gain access to far-off bases and ports and cement links to foreign military establishments.

High-level disillusionment with U.S. efforts to get other countries to restrain their arms traffic also was growing long before the Reagan election. During the Carter presidency, France's sales of arms to Third World countries rose from $1 billion to $8 billion, while American sales held constant at slightly more than $9 billion. The Soviet Union also increased its share of the Third World arms market. Its shipments rose in value in the Carter years from $6.5 billion to $14.9 billion.

The desire of the U.S. arms makers to penetrate Third World markets denied to them in the 1970s coincides with what observers say is a spreading fever in those markets to obtain the latest military gadgets. In one country after another, the military appears to be succumbing to the lure of new technology.

"To be quite honest, one reason we want F16s is prestige," said a top Venezuelan about his country's pending purchase. "What do you expect us to buy, bows and arrows?"

Nevertheless, the removal of some restraints operative in the Carter period has evoked sharp criticism, especially in Congress.

House Foreign Affairs Committee Chairman Clement J. Zablocki (D-Wis.), warning of a potential escalation in the conventional arms race in the Third World, cautioned last October that "if we do sell [F16s] to Pakistan, you can bet your bottom dollar every other country will want the same treatment, and it won't stop with Venezuela."

Last week the House Appropriations Committee turned down an administration request for $301 million in extra 1982 budget funds to pay for emergency military aid increases to El Salvador, Honduras, Somalia, South Korea, Thailand, Morocco, Tunisia, Portugal, Spain and Turkey. The 1983 aid request, still to be adopted, already has been rewritten drastically in committees to do away with the low-interest credits the administration wanted to provide to many countries.

Critics of the more aggressive policy make a number of points.

They warn that by introducing advanced military technology into one country the United States sows the seeds of regional arms races and increases the chances of fighting between U.S. friends. This, in turn, can ensnare Washington in a briar patch of distant parochial rivalries.

India and Pakistan used U.S. weapons against each other in their 1965 and 1971 wars. Turkey used American arms in its 1974 invasion of Cyprus, and more recently Argentina and Britain fought each other with U.S.-supplied A4 aircraft and Sidewinder missiles.

The proposed U.S. sale of F16s to Venezuela has drawn an official protest, not from Soviet-backed Cuba, but from Venezuela's neighbor, Guyana. Venezuela has a claim on 65 percent of Guyana's territory, and Guyanese leaders apparently fear that the new weapons could embolden the Caracas government to back up its claim with force.

"We supply these weapons on the assumption that they'll be used against our enemies, but almost invariably they're used against their enemies," said Michael Klare, a fellow at the Institute for Policy Studies specializing in arms issues.

Critics also question the long-range financial and economic impact on countries that purchase the new generation of military electronics.

Even Venezuela, a country rich with oil money that plans to pay cash for the F16s, faces economic problems, such as sluggish oil sales and growing debt.

For poorer countries, the problems connected with arms purchases are far more severe.

In Congress, liberals opposed to military aid and conservatives concerned about deficits have given the administration a small "no frills" program of military grants and a much larger loan program that appeals to budget-cutters now but, some claim, could buy trouble later.

The loans, earmarked for the purchase of U.S. military items, are borrowed through the Treasury, and don't show up in the budget appropriation. But since the recipients pay near market interest rates, the General Accounting Office has questioned whether some of these countries will be able to keep up with payments without severe strains.

Sudan is an example of the dilemma. For two years in a row the administration, citing the need to buttress the regime of President Jaafar Nimeri against a Soviet-backed threat from Libya and Ethiopia, has asked Congress to support a $100 million military aid program to enable Sudan to buy U.S. M60 tanks, additional F5 aircraft, radar and anti-aircraft weapons. Because Sudan faces "grave economic difficulties," the administration urged that the money be given as grants or low-interest loans.

But Congress provided only $25 million as grants, and rejected low-interest loans. Instead, it allowed Sudan to borrow the additional $75 million through the U.S. Treasury at near market rates.

Critics say that Nimeri's prestige is now tied up with a military buildup backed by the United States, but the country's economic situation makes it questionable whether it can afford hefty loans at high rates.

"We are stepping up military aid and reducing economic aid to countries that are poorer than church mice," warns a congressional aide. "It's all going to come home to roost some day."

The main administration argument for the program is the need to back regimes faced with a Soviet, Cuban or Libyan threat.

Typical of this is the administration's justification for selling Venezuela F16s. Testifying before a House Foreign Affairs subcommittee last October, Assistant Secretary of State Thomas O. Enders cited the "buildup in Cuba," including the presence of Mig 23 aircraft, two submarines and the Koni-class surface ship, as presenting a "potential threat to all the countries that border on the Caribbean." Enders warned that the Cuban-based Mig 23s, which do not have the range to strike Venezuela, "could be very substantially extended by the use of airfields in Grenada and Nicaragua."

But Venezuelan officials deny that they want the planes to combat a Cuban threat, which they say is nonexistent. And American officials privately acknowledge that the real reasons behind arms transfers are far more complex.

"Venezuela wants these planes," one aide commented. "It's difficult to say to a sovereign country with which we have friendly relations, 'You're not worthy to have the same thing everybody else has.' "

Critics of the military aid program often suggest that it is part of a grand imperial scheme to extend American influence worldwide. U.S. officials acknowledge that it is a crucial means for winning friends and bargaining for base rights and emergency access for the U.S. military in countries from Spain to Oman.

In its 1983 Kenyan aid request to Congress, for instance, the administration noted that "Kenya permits U.S. military access to its facilities," and "has agreed to U.S. military construction projects to improve its facilities which the U.S. utilizes."

Other officials suggest, though, that the most troublesome part of the military aid program is that it often seems to be haphazard, driven by an array of non-strategic considerations. Taken together, many decisions influenced by many different parts of the U.S. governmental bureaucracy can have the effect of proliferating extremely lethal weaponry to every corner of the world.

In early July, the State Department hurriedly notified Congress that the government intended to sell Singapore $30 million in mortar-locating radars.

Afterward, officials said the request had been pending for months but was expedited to coincide with the U.S. visit of Singapore Prime Minister Lee Kuan Yew. In South America, a major factor in the administration's "case-by-case" approach to decisions on arms sales is growing pressure to supplant European and Soviet arms manufacturers that have grabbed most of that market with the sale of more than 110 supersonic fighters between 1973 and 1980.

"Of course we are not in the business of selling aircraft," Richard Burt, director of the State Department's Bureau of Politico-Military Affairs, testified last year.

Nevertheless, the Reagan administration clearly wants an arms sale policy that helps those who are in that business. Last year it specifically revoked a Carter order, nicknamed the "pariah order" by arms traders, barring U.S. embassies abroad from promoting arms deals or helping arms merchants without top-level authorization.