Alterations in nitric oxide (NO) biosynthesis in the heart have been implicated in the pathophysiology of heart failure. We compared changes in cardiac nitric oxide synthase (NOS) activity and expression in genetically heart failure-prone (SHHF) rats at 6, 12, and 18 mo of age with those in age-matched spontaneously hypertensive (SHR) and Sprague-Dawley (SD) rats. Systolic blood pressure was significantly higher in SHHF and SHR rats compared with SD rats; however, it declined with age in SHHF rats only. Left ventricular mass increased with age in SHR and SHHF, but not in SD rats. Plasma nitrate and nitrite level was elevated in SHHF and SHR rats at 18 mo only. In left ventricular homogenates from SHHF rats, Ca(2+)-dependent NOS activity increased markedly with age and was accompanied by enhanced expression of endothelial NOS (eNOS). In contrast, SHR rats showed a much smaller increase in Ca(2+)-dependent NOS activity over time without changes in eNOS expression; neither parameter was altered with age in SD rats. Ca(2+)-independent NOS activity was not detected in any heart. This is the first report of a unique alteration in myocardial NOS activity in hypertensive rats genetically prone to heart failure.