Myocardial constitutive and inducible nitric oxide (NO) synthase activities were measured in right ventricular tissue from 17 patients with dilated cardiomyopathy (DCM). A significant activity of inducible enzyme was accompanied by a low activity of the constitutive NO synthase. Thus, the myocardium has the capacity to express both NO synthases. NO may have a physiological as well as a pathological role in the human myocardium.