While 3,000 American sailors steamed up and down the Persian Gulf last month, 10 congressmen headed for a cooler August on the Arctic tundra. Both missions had to do with securing oil supplies. The congressmen's task was to check out the Arctic National Wildlife Refuge. The administration wants to explore it for oil. According to current estimates, there is a 20 percent chance of finding an oil field there as huge as the one at Prudhoe Bay, 60 miles to the west, which now provides Americans with one out of every five domestically produced barrels of oil.

Ecologists, however, worry about the damage that oil exploration might do to the wildlife refuge. Their fears have been heard before. When Prudhoe was discovered, environmentalists protested that the pumping and the pipeline would shatter the delicate ''ecosystem.''

They were wrong. Even the foremost congressional opponent of Arctic exploration, Rep. Morris Udall, admits it. ''We've had 15 years or so with Prudhoe and we came out pretty good,'' Udall concedes. ''The people who talked about ecological disaster have been proven wrong.'' So? ''But 15 years isn't very long in terms of something as fragile and precious as this Northern Slope.''

How many years do we wait? Fifty? The question is important and the issue pressing because, even after a decision to explore is taken, it will be between 10 and 15 years before any oil starts to flow. During a future oil shortage, we will not be able just to turn an Alaskan spigot. That capacity has to be built now.

The main concern of environmentalists is the Porcupine caribou herd, which numbers about 180,000 and migrates to the coastal plain for calving. But caribou concern was raised about Prudhoe 15 years ago. And it turns out that the caribou did very well, thank you. Their numbers have tripled since the pipeline was installed. It is a paradox of the ecology movement that its central theme is the astonishing creative adaptability of an interdependent Nature, yet its central task is to prevent man from disturbing the current natural balance lest Nature collapse from the strain.

One obvious way to reconcile national security with environmental concerns is strictly to regulate development. No drilling during caribou calving season, if you like. One proponent of the environmental view, writing in The New York Times, warns against such compromises, citing ''precautions gone for nought'' at the Arctic National Wildlife Refuge.

One incident, it seems, ''occurred in March 1986. Glenn Elison, manager of the wildlife refuge, reported that a female polar bear had been routed from and abandoned her den when an oil company crew inadvertently transported equipment through the restricted area. Again, elaborate precautions proved faulty.''

What to say to those who rank energy independence with polar-bear housing on the national agenda? Ultimately, sentimental environmentalists are concerned less about the real environment than about the environment of the imagination. People want to know that pristine places exist -- somewhere -- even if they will never see them. No doubt, such inaccessible preserves are a soothing social asset.

But Alaska consists of 375,296,000 acres. The Arctic National Wildlife Refuge contains 19,000,000 acres. The area proposed for exploration is 15,000 acres, or less than 1 percent of that. It is hard to see how the lower-48 urban dweller's idea of the Great North is noticeably damaged by the existence of gravel pits and oil rigs in this dot in the wilderness. Has our idea of the Great North been diminished by the fact that some 5,000 acres of Prudhoe Bay have been given over to industry?

Environmentalists correctly point out that the Reagan administration, now touting Arctic exploration in the name of energy independence, has prevented other steps toward that goal. It resisted energy-efficiency standards in electrical appliances. It slackened fuel-efficiency standards in cars. It steadfastly opposes oil-import fees and gas taxes. And as part of its budget balancing flimflam last year, it proposed slowing down filling the Strategic Petroleum Reserve.

But the fact that the administration has been lax on conservation does not refute the argument for development. Administration sincerity is irrelevant to the case for exploration. The facts remain: American energy dependence has grown dramatically during the 1980s oil glut lull. Almost half of American oil (45.6 percent in July) is now imported. There may soon be dead Americans in the Persian Gulf. And in the final analysis, when Americans die there, they die for oil. Domestic American oil production is declining. The Prudhoe reserves will be gone within 10 to 20 years. The Arctic National Wildlife Refuge holds the promise of replacing that flow.

Apocalyptic predictions about the caribou were wrong before. The weight of the evidence is that they are wrong again. But even if they are right and one has to choose between caribou and country, it is hard to see how there is a choice.