Faced with a nationwide slump in domestic air passenger and cargo traffic, Baltimore-Washington International Airport is looking increasingly to the international market for its growth.

In the past two years alone, the number of international passengers passing through the airport grew 77 percent and international cargo more than doubled, according to figures released yesterday by Maryland Secretary of Transportation O. James Lighthizer.

"As international markets open up at BWI, the region's businesses are filling up seats and cargo holds," he said, "and these are particularly strong gains in light of economic conditions."

Although international traffic constitutes only 9 percent of BWI's total passenger traffic and 14 percent of its cargo traffic, those are expected to be the growth areas in the years ahead, said Ted Mathison, head of the State Aviation Administration, which operates BWI.

In 1991, 743,337 passengers traveled abroad from BWI, 35.2 percent more than in 1990. International cargo totaled 35.8 million pounds last year, an increase of 24.6 percent, the data showed.

By contrast, the number of domestic air passengers declined 5.4 percent, and domestic freight traffic was down 7.5 percent. Total passenger traffic was down 3.2 percent for 1991, and total cargo was down 3.9 percent.

The increase in international traffic helped boost the airport's operating profit of $17.8 million in the fiscal year that ended June 30, 1991.

Mr. Mathison said the rise in international passengers also reflects more intense efforts to attract international carriers. In the past two years, BWI has added three foreign carriers -- KLM Royal Dutch Airlines, Icelandair and Ladeco Chilean Airlines. USAir, BWI's largest carrier, also has increased its international service by adding routes to Montreal and Ottawa.

Demand for international service was especially high at the end of the year, Mr. Mathison noted. More increases are expected in international routes in 1992 with the planned addition of Cayman Airways and an expansion of Icelandair routes.

Domestic cargo for the year was 209.7 million pounds, and there were 9.1 million domestic passengers.

In all, 245.5 million pounds of cargo and 9.9 million passengers moved through BWI.