For 13 years, the Organization of Economic Cooperation and Development, an international group staffed largely by scientists and technical experts from 24 nations, plodded toward solving the mutual problem of how to protect citizens from the hazards of new chemicals.

Last year, just as the scientists were close to resolving a key issue, the United States threw a monkey wrench into the process.

With a Dow Chemical Co. executive whispering in the chief negotiator's ear, the United States forced the group to back away from a mandatory set of tests on chemicals before they enter the marketplace. Eventually, the panel adopted a non-binding recommendation that its European members regard as meaningless.

Congress is investigating Dow's role in the negotiations as part of a continuing probe into whether industry views unduly influenced policies of the Environmental Protection Agency and its enforcement of environmental laws.

But while Dow's involvement in this episode has drawn attention, the fate of the chemical testing system is part of a larger development. Since Ronald Reagan became president, the United States has blocked, damaged or destroyed numerous international environmental efforts in a sometimes clumsy campaign to remove regulatory obstacles, domestic or international, from the paths of commerce.

"A change has taken place," Dr. Rune Lonngren, a Swedish delegate to the OECD, said last week. "We have been concerned, of course. The outside world needs to share the resources the United States has . . . . We hope this funny story was isolated."

Environmentalists, scientists and officials in the EPA and other government agencies say the OECD case is far from isolated.

They offer, among other examples:

* In March, 1981, six weeks after it took office, the Reagan administration launched a campaign to torpedo an international Law of the Sea Treaty crafted under eight years of Republican and Democratic administrations.

The treaty, which establishes an international code for exploitation of the world's ocean resources, was ratified overwhelmingly in a United Nations vote and signed by 120 nations. The United States, concerned that the treaty would restrain deep-sea mining operations, refused to sign it and persuaded some of its key allies, including Britain and West Germany, to go along.

* The administration has remained steadfastly opposed to new controls on air pollution to curb the transboundary effects of acid rain, prompting Canada to charge the United States with negotiating in bad faith under a 1980 agreement that called for a diplomatic solution to the problem.

* Last fall, the United States cast the only "No" vote on a proposed U.N. World Charter for Nature. More than 135 other nations voted for the charter, which would have committed U.N. members to "respect nature and its essential processes."

* Less than two months later, the United States cast the only negative vote among 147 nations on a U.N. initiative to prohibit export of banned hazardous products without the knowledge and consent of the recipient country.

President Carter, by executive order, had prohibited such exports without advance notice of the substance's potential effects. Reagan revoked Carter's order 17 days after he took office.

* The administration has attempted to cut U.S. funding to the U.N. Environmental Programme to $3 million, 70 percent below its 1980 level. Congress restored most of the funds in fiscal years 1982 and 1983, but the administration is trying again in fiscal 1984.

* Budget cuts also threaten several research programs with an international emphasis. In fiscal 1984, for example, the administration has proposed to end research in Antarctica and the U.S. contribution to UNESCO's Man and the Biosphere research program.

The administration's critics contend that the frequent reversals in longstanding U.S. positions threaten to cost this nation its historical leadership role in efforts to find solutions to international environmental problems.

A study funded by the German Marshall Fund reported last month, for example, that the United States is alone among western nations in moving away from more stringent controls of sulfur dioxide emissions, believed to be a major component of acid rain.

In the case of the U.N. nature charter, critics of the U.S. position concede that the document needed rewording but contend that the United States, as the only naysayer, gave up an opportunity to mold a flawed document into a sound one.

"It was a crass disregard of the sense of the General Assembly. They were just throwing away the U.S. stance of gentle persuasion, of leadership among other countries in a direction where we know we'll have to go to survive," said Barbara J. Bramble, international programs director for the National Wildlife Federation.

Similarly, the United States effectively pulled itself out of the debate over the Law of the Sea Treaty last December, when Reagan announced that the United States would not pay its share of the cost of drawing up rules under the treaty. The United States was the only major western nation to take that step.

European nations, even those that joined the United States in refusing to sign the treaty, "are not likely to join the U.S. in totally rejecting the international process," said Lee Kimball, director of Citizens for Ocean Law, an independent group that has monitored the treaty process.

The administration's direction has recently been criticized within the government. Included was a sharply worded speech last month by an official of the office of the U.S. trade representative, who contended that State Department appointees were bumping technical experts off international delegations at the request of industry.

"We can't help industry, or foreign nations, from the viewpoint of the United States if at the same time industry is working to knock our experts off the delegation," said the official, Don Abelson, who coordinates industry and government positions on international regulatory efforts.

"If you take your experts off, you leave people who spout nice words, but you don't get anything done. So far, all the administration has accomplished is to provide a temporary halt in the process."