The role of cyclosporine (CSA) alone or in combination with various chemotherapeutics in the development of renal toxicity was evaluated in rats. Administration of 20 mg/kg/day CSA for 4 weeks caused renal functional and structural changes similar to those reported in man. The combined administration of CSA and various chemotherapeutic drugs with a nephrotoxic potential, such as gentamicin (at therapeutic doses), amphothericin B and ketoconazole, which are frequently used in immunosuppressed patients, did not aggravate the CSA induced toxicity in the rat model. Gentamicin at toxic doses, however, increased CSA nephrotoxicity. Thus, the nephrotoxicity induced by CSA has a different pathogenetic mechanism.