OBJECTIVE Recent studies indicate that endothelial type nitric oxide synthase (NOS3) modulates cardiac systolic and diastolic function and the inotropic responsiveness to beta-adrenergic agonists, and may affect myocardial oxygen consumption. Although NOS3 is a constitutive protein, its levels of expression can be modified by various physiological and pathophysiological stimuli. We investigated whether the cell-specific expression of NOS3 mRNA and protein are altered in cardiac hypertrophy. METHODS Left ventricular cardiac myocytes and coronary microvascular endothelial cells were freshly isolated from 12 week old male spontaneously hypertensive rat (SHR) and matched normotensive Wistar rat hearts. NOS3 protein levels were assessed by Western analysis, and mRNA levels by RT-PCR and Southern blotting. RESULTS Left ventricular/body weight ratios were significantly increased in SHR compared to Wistar controls, indicating significant hypertrophy. The levels of NOS3 protein were markedly decreased in SHR compared to Wistar cardiac myocytes (by approximately 85%). By contrast, the expression of NOS3 mRNA normalized for GAPDH was increased approximately 3 fold in SHR cardiac myocytes relative to Wistar controls. In freshly isolated microvascular endothelial cells, however, levels of NOS3 protein and NOS3 mRNA were similar between the two groups. CONCLUSIONS The expression of NOS3 is selectively altered in cardiac myocytes but not coronary microvascular endothelial cells of young SHR hearts, with a marked decrease in NOS3 protein but an increase in NOS3 mRNA. This dysregulation of NOS3 could contribute to contractile dysfunction in left ventricular hypertrophy.