Morphological changes in the myocardium after left ventricular hypertrophy, due to chronic experimental hypertension, require an understanding of the quantitative relationship between myocyte and nonmyocyte compartments forming the structural framework of the myocardium. Hypertension was induced by long-term low-dosage inhibition of nitric oxide synthesis by NG-nitro-L-arginine methyl ester (L-NAME) in rats. L-NAME (12 mg/kg) was given to animals in water ad libitum during 15 weeks. After this period, systolic blood pressure increased almost 50% as compared with that in the control group. Morphological changes in control and L-NAME animals were investigated with stereology and immunohistochemistry. Comparing control and L-NAME animals, the surface density of myocytes decreased 73.7% while the mean cross-sectional area increased 97.6% in L-NAME rats. The volume density of myocytes decreased 45.9% and the volume density of the interstitium increased 71.7% in L-NAME rats. No stereological difference was found in blood vessels comparing the two groups. Remodeling of the cardiac interstitium occurred with increased deposition of both fibronectin and type III collagen. Fibronectin was seen in both early and latter responses to infarction while type III collagen was seen mainly in areas of incomplete healing among myocytes and around intramyocardial branches of the coronary arteries. The long-term low-dosage administration of an inhibitor of the NO synthase such as L-NAME causes myocyte hypertrophy and early interstitial and perivascular fibrosis without important quantitative changes in microcirculation.