AIM: To investigate the anti toxic effect of iminoethyl lysine (Imi), an inhibitor of inducible nitric oxide synthase (iNOS), on the guinea pig renal injury with gentamicin (Gen). METHODS: The experiment was consisted of four groups: normal animal group, gentamicin model group (Gen group), Gen+Imi group and Imi group. Guinea pigs of both Gen group and Gen+Imi group were given (sc) Gen 100 mg?kg -1 ?d -1 for 10 d, and the guinea pigs of Gen+Imi group were treated with Imi intraperitoneally. Gen group were treated with the same volume of saline. Guinea pigs of normal animal group were treated with subcutaneous and venous injection of the same volume of saline. Guinea pigs of Imi group were treated with the same dose of Imi. The renal function of guinea pigs of all groups was examined before and after experiment. Renals of all groups were embedded with paraffin, sectioned, stained with HE, and examined with the light microscope. iNOS was examined in the kedney of all groups with immunohistochemistry. RESULTS: The expression of iNOS was negative in kidney of normal animal group and Imi group, and positive in both Gen group and Gen+Imi group. The activity of iNOS of Gen group was significantly stronger than that of Gen+Imi group. The damage of renal of Gen group was much more severely than that of Gen+Imi group. CONCLUSION: The expression of iNOS is positive in the renal damaged with Gen. Imi can significantly inhibit the activity of iNOS, and has also the significant anti toxical effect on the renals damaged with Gen. It suggests that nitric oxide takes part in the process of pathology of renals damaged by gentamicin.