Although OPEC nations are believed to hold approximately 80 percent of its bonds, the Federal National Mortgage Association told Congress yesterday it neither knew - nor cared to know - how much Arab money was being invested in its securities.

Robert Bennett, executive vice president and chief financial officer of the government-sponsored corporation that generates housing funds by buying mortgages from lenders, testified that he saw no valid reason for seeking to know how much foreign money was invested.

Fannie Mae records show only the street names of the brokers who buy on behalf of unnamed clients. Bennett fears politically sensitive oil producers might take their petrodollars elsewhere were the true ownership to become known.

Bennett surprised the House subcommittee by revealing a list of 22 meetings with OPEC representatives. These included 13 bilateral meetings with officials of the Saudi Arabian Monetary Agency, that country's central bank.

Seven trips to Saudi Arabia were recorded. At another five international gatherings, such as World Bank meetings, Bennett and Fannie Mae chairman and president Oakley Hunter met OPEC representatives. There were two meetings with Venezuela, and one each with Kuwait and Bahrein.

Ruth T. Prokop, former general counsel of the Department of Housing and Urban Development and Fannie Mae director, said the board heard about "only one or two trips," and the exact purpose of the meetings never was discussed in detail. Board members did not ask, nor were they told, the identity of investors.

Bennett told an incredulous subcommittee chairman, Rep. Benjamin Rosenthal (D-N.Y.), that no minutes were even taken of any of the meetings nor were bond purchases ever discussed. At one point Rosenthal exclaimed, "You must be the only businessman in the world who doesn't know how well his product is selling!"

Rosenthal's committee has been conduting a series of hearings for the purpose of ascertaining the amount of foreign, mainly OPEC, investment in this country. He complained yesterday that the Treasury had been unable to supply figures.

A Treasury spokesman said the amount invested in government agency bonds by eight Middle Eastern countries was $3.8 billion at the end of 1978. Most of that is invested in Fannie Mae, the second largest issuer of debt instruments after the Treasury. Fannie Mae's total outstanding debt stands at $46 billion.

The Treasury has a country-by-country breakdown but is prohibited from making it public under a 1976 law. This law shields the identities of individual investors; for this purpose, a government is considered an individual. And since 90 percent or more of Middle Eastern investments are made by their governments, the Treasury says it cannot disclose these countries' holdings.

The Spokesman admitted OPEC countries had made known their political sensitivity about exposure to the U.S. government. In fact, Bennett said Fannie Mae had negotiated a $600 million sale of debentures to the Saudi Arabian Monetary Agency in 1974. Soon thereafter, Fannie Mae was told by the Treasury that foreign investors should henceforth make their purchases through normal markets; i.e., through third parties that hold the securities in street names.

OPEC's investment in Fannie Mae, plus other agency and corporate stocks and bonds, or $12 billion represents only about a quarter of OPEC's total $50 billion investment in America, according to the Treasury.