Different nitric oxide synthase (NOS) isoforms are found in the kidney. Some studies provided evidences that increased endothelial NOS (eNOS) activity leads to restoration of renal function after injury, but activation of inducible NOS (iNOS) aggravates renal failure. In the present study, the beneficial effects of selective iNOS blockade in gentamicin (GM) induced nephrotoxicity have been investigated. Four groups of rats were studied. Untreated control rats received saline. In GM group, GM was injected (IV, 4 mg kg(-1)). In GM + L-NAME group rats received L-NAME (N-omega-L-arginine methyl ester, a non-selective NOS inhibitor) simultaneously with GM (IV, 30 mg kg(-1)). Additional doses of L-NAME were administered 2 and 4 h after GM (IP, 30 mg kg(-1)). In GM + L-NIL group rats were treated by N-imino-ethyl lysine (L-NIL, a selective iNOS inhibitor). First dose (IV, 3 mg kg(-1)) administrated simultaneously with GM. Next doses (IP, 3 mg kg(-1)) were administered 2 and 4 h after GM. In all groups, serum and urine creatinine levels were measured. Creatinine clearance was calculated and considered as an estimation of glomerular filtration rate (GFR). Urine N-acetyl-b-D-glucose aminidase (NAG) activities were also determined. After experiments, kidney sections were histologically studied. Selective iNOS inhibition by L-NIL prevented the GM-induced decrease in GFR and increase in creatinine levels, while complete non-selective NOS inhibition by L-NAME aggravated the GFR reduction, elevation of creatinine levels and enzyme release (P < 0.05). Histological studies showed that GM-treated kidneys had evidences of tubular damages and these damages were less evident by the administration of L-NIL. In conclusion, selective inhibition of iNOS may prevent GM-induced nephrotoxicity, whereas non-selective inhibition of NOS aggravates it.