Commerce Secretary Malcolm Baldrige will lead 34 business leaders on a seven-day mission to the Middle East next week to try to preserve America's position as a leading trading partner of Saudi Arabia and to open new markets in Algeria.

Baldrige said his presence on the trip is part of Reagan administration policy to help American business interests promote exports, especially in the Mideast, where other countries frequently send trade delegations headed by government ministers. "We've left it all to the private sector," he said.

He said the business delegation is composed of top corporate executives who have the authority to close deals on the spot. The chairmen and chief executive officers of both Boeing and McDonnell Douglas, which are in direct competition with the European Airbus maker for sales to overseas airlines, signed up for the trip even though both companies have carved out their own contacts in the area. Baldridge said they want to sell their planes to national airlines of Saudi Arabia and Algeria.

The trip also includes small and medium sized firms, many of which never before have tried for overseas business.

Saudia Arabia, which Baldrige described as "a voice of moderation in the Middle East" that for years has maintained "a special relation" with the United States, is this country's sixth largest export market. But the U.S. share of the Saudi market has declined in the past two years, from 25 percent to 20 percent, in the face of strong competition from other industrialized nations.

Even so, U.S. exports of more than $7.3 billion in 1981 increased by 26 percent over 1980 as the oil-rich kingdom spent itself into the position of becoming the world's 12th largest import market.

Baldridge called Algeria, which also has vast oil and natural gas resources, "a great potential market for U.S. business" that barely has been scratched. The United States supplies about 7 percent of Algeria's $10 billion in imports.

The mission was restricted to firms in six fields that officials felt held the greatest potential for increased trade with Saudia Arabia and Algeria -- communications, water resources technology, agribusiness, aviation and avionics, manpower development and operations and maintenance services. Business leaders are paying about $12,000 each as their share of the trip, which starts Dec. 1 and ends Dec. 8.