RESUMEN
<p><b>OBJECTIVES</b>To determine the expression of nitric oxide synthase (NOS) in testis and to investigate the effects of NO on the reproductive function of testis.</p><p><b>METHODS</b>Testes of adult male Sprague-Dawley rats were fixed in 4% paraformaldehyde. The paraffin sections were made as routine. Immunohistochemical ABC method was used to observe the localization of NOS.</p><p><b>RESULTS</b>Endothelia NOS (eNOS), neuronal NOS (nNOS) and inductive NOS (iNOS) were all expressed in Leydig cells. Only eNOS was expressed in peritubular myoid cells, endothelial and smooth muscle cells of blood vessel, while only nNOS expressed in tunica adventitia of testicular blood vessels. The reactive substance distributes in cytoplasm with negative nuclei. Immunoreactivity for eNOS, nNOS and iNOS in all spermatogenic cells was negative.</p><p><b>CONCLUSIONS</b>Three kinds of NOS were all expressed in testis and the distribution of different NOS had a little difference.</p>