In kidney, nitric oxide (NO) synthesized by nitric oxide synthase (NOS) regulates sodium and water excretion, and renal medullary blood flow. The expression of constitutive NOS, endothelial NOS (eNOS) and neuronal NOS (nNOS), were assessed in kidney of the spontaneously hypertensive rat (SHR) and the normotensive Wistar Kyoto (WKY) rat by Western blot analysis and immunocytochemistry. Neuronal NOS expression was observed in the cortex and eNOS was detected only in theinner medulla of both WKY and SHR. In SHR, expression of eNOS was attenuated to 35.1 +/- 10.8%, while expression of nNOS was only 57.5 +/- 5.7% of the levels seen in WKY rat. Immunocytochemical studies revealed decreased staining of nNOS in the macula densa, collecting ducts and in the glomerulus of SHR compared to WKY rat. Endothelial NOS immunoreactivity was restricted to vascular structures of the inner intima cells and smooth muscle cells, and was markedly reduced in the vasculature of SHR. The decreased renal blood flow observed in SHR may be linked to a diminished expression of eNOS and nNOS, underscoring the importance of these enzymes in the pathophysiology and maintenance of genetic hypertension.