Maryland National will pay $438 million for a bank.

The combined assets of Baltimore-based Maryland National Corp. and Washington-based American Security Corp. will be about $12.9 billion, according to a joint statement. The merger is subject to regulatory and shareholder approval and is expected to be consummated in the first quarter of 1987. Maryland National is Maryland’s largest bank holding company, with assets of $8.6 billion as of June 30. American Security is Washington’s second-largest bank holding company, with assets of $4.3 billion as of June 30.