Psychoactive drugs provide essential intervention in the care of transplant recipients, yet little is known of their interaction with immunosuppressants such as cyclosporin (CSA). Lithium (Li) is an invaluable drug for the treatment of manic disorders in organ transplant patients. As both these drugs are known to produce renal toxicity, the concomitant use of CSA and Li may be potentially harmful. The present study was undertaken to investigate the effect of CSA and Li chloride individually and in combination on renal structure and function of rats. Male Sprague-Dawley rats were divided into the following eight groups of seven animals each: group I, control (vehicle only); group 2, Li (2 mEq/ kg i.p.) alone; group 3, CSA 12.5 mg/kg (subcutaneous); group 4, CSA 25 mg/kg; group 5, CSA 50 mg/kg; group 6, CSA 12.5 mg/kg + Li; group 7, CSA 25 mg/kg + Li; and group 8, CSA 50 mg/kg + Li. The drugs were given once a day for seven days; Li being administered 30 min before CSA. Twenty four hours after the last dose of drugs the animals were sacrificed and blood samples were analyzed for blood urea nitrogen (BUN), serum creatinine (SCr), CSA and Li levels. The left kidney was analyzed for malondialdehyde (MDA) and conjugated dienes (CD) levels and right kidney was used for histopathological studies. Our results showed that Li alone did not produce any significant renal toxicity, whereas CSA dose dependently caused structural and functional changes in kidneys. However, significantly higher structural and functional impairment was observed in the animals treated with Li plus CSA as compared to CSA alone treated animals. Several fold increase in blood Li level was also noticed in the rats concomitantly treated with CSA and Li. A significant increase in MDA and CD in the rats treated with CSA plus Li suggests the role of oxidative stress in drug induced nephrotoxicity. These findings clearly demonstrate that even non toxic doses of Li may significantly exacerbate CSA induced nephrotoxicity in rats. The enhanced nephrotoxicity following concomitant use of these drugs may be attributed to significant increase in the bioavailability of Li and enhanced oxidative stress. Further clinical studies are warranted to investigate the interaction of these nephrotoxic drugs in human subjects.