Nitric oxide synthase, the enzyme responsible for the synthesis of nitric oxide and citrulline from arginine, was potently inhibited by the anthracycline antibiotics doxorubicin (Ki = 24 microM) and aclarubicin (Ki = 50 microM). These drugs were non-competitive inhibitors with respect to arginine. This action on nitric oxide synthase may explain some of the cardiovascular and cytotoxic actions of these chemotherapeutic drugs.