T hese studies were performed to determine if the effects of angiotensin II infusion on the development of cardiac fibrosis could be modified by the chronic inhibition of nitric oxide synthase activity. NG-nitro-L-arginine-methyl ester (L-NAME) was administered to adult Wistar rats in drinking water (40 mg/kg per d). Although blood pressure was maintained at hypertensive levels after 2 wk, cardiac hypertrophy or fibrosis did not occur. Angiotensin II, given for 3 d at a dose which induced little or no blood pressure elevation and minimal if any fibrosis, caused significant fibrosis when given to a rat pretreated for 2 wk with L-NAME. This marked fibrosis did not occur if angiotensin II was given shortly after L-NAME treatment was begun or briefly after discontinuation of L-NAME. The fibrosis that occurred with combined treatment was characterized by increased immunodetectable fibronectin, the presence of inflammatory cells within interstitial and perivascular regions, and increased steady state mRNA levels for matrix genes and atrial natriuretic protein. The data indicated a regulatory role for nitric oxide in modulating the angiotensin II-induced cardiac fibrosis and suggest a potentially important autocrine or paracrine role for nitric oxide in fibroblast proliferation.