The U.S. government, apparently acting on the advice of a Dow Chemical Co. executive, last year effectively blocked an international system of chemical testing that would have required that all new chemicals be screened for their long-term toxic effects.

The action came last December, when the Organization of Economic Cooperation and Development voted to adopt a voluntary and well-diluted version of a testing system that the group had been studying for several years. The system, already in place in European Common Market countries, requires a series of tests on a chemical's makeup and its health and environmental effects before it can be marketed.

Under the Carter administration, U.S. delegates to the international organization strongly supported making the testing system mandatory. Had the vote last December been a recorded one, the United States would have voted against even the non-binding recommendation, according to sources familiar with the session.

Current and former Environmental Protection Agency officials say the change in the U.S. position on the testing system had been building for more than a year. But they said the new position was solidified at a meeting in Paris last June after Donald D. McCollister, a Dow executive serving as a private-interest adviser to the delegation, briefed the U.S. ambassador to the organization.

The briefing session excluded both Irving Fuller, the EPA's official delegate to the meeting, and Khristine Hall, an Environmental Defense Fund lawyer who was serving as a public-interest adviser.

Dan Wakefield, a Dow spokesman at the company's headquarters in Midland, Mich., confirmed that McCollister had briefed Ambassador Abraham Katz in Paris "within his role as an adviser." But a June, 1982, memo to EPA Administrator Anne M. Burford from Richard Funkhouser, the agency's director of international affairs, said McCollister gave Katz "an earful in a private conversation" with "revealing results."

Katz "now understands the need to reverse the previous administration's drive to change U.S. laws and regulations through decisions taken in international organizations . . . , " Funkhouser wrote. Funkhouser urged Burford to attend an October meeting of the group, saying "you personally must inform your international peers of the administration approach to chemicals, if we are to succeed in changing directions." Burford met with the group in Paris six weeks before the December vote.

Wakefield said McCollister gave the ambassador "the chemical industry view of OECD activities." Dow's position, he said, is that the U.S. approval system for new chemicals under the Toxic Substances Control Act is sufficient to protect the public.

But Rep. James H. Scheuer (D-N.Y.), chairman of a House Science and Technology subcommittee that is investigating Dow's role in the international meeting, said the memo indicates that the chemical industry, not satisfied with taking control of the EPA itself, is using the agency as its agent to affect public policy.

Fuller, who has since been detailed to the office of the U.S. trade representative, declined to comment.

Hall said it was "very clear that the ambassador was taking one side of the issue," and she charged that the United States' refusal to adopt the testing rules "means essentially that European citizens will be better protected than Americans."

A delegate from Sweden to the international organization, Dr. Rune Lonngren, said in an interview this week that the United States "officially acted through normal channels, but I guess McCollister had a say in the U.S. viewpoint. He has made clear to me that he is not in favor of the testing rules, believing they are unnecessarily cumbersome for industry."

Lonngren, who was in Washington to testify before a House subcommittee considering reauthorizing the toxic substances law, said he was "surprised" when he discovered in 1981 that the United States wanted to reverse its longstanding support for the testing rules.

"More and more it came out that the United States would not like action" on the rules, he said. "We wanted a decision paper, a binding paper. When it turned out that was not possible, we tried for a recommendation that would be morally binding but not legally binding." He said he was surprised when the United States did not support that either.