FK506, a newly developed immunosuppressive agent, has recently been used for human liver and kidney transplantation. The present study was carried out to assess the functional and morphological changes by acute or chronic administration of FK506 in heminephrectomized rats. FK506 was given intravenously at the dose of 0.384 mg/kg/hr for 90 min in acute experiment. FK506 was administered by gastric gavage at doses of 1, 2.5, 5 mg/kg for 21 days. Blood and urinary biochemistry were monitored every week. Inulin and PAH clearance studies were conducted during the infusion of FK506 for acute study, and at day 21 for chronic study. Some of the rats were treated with diltiazem (Dilt), captopril or prazosine for 21 days to prevent FK506 nephrotoxicity. Acute infusion of FK506 did not change renal and systemic hemodynamics. Creatinine clearance showed a dose dependent decrease by 10-20% in the rats with chronic administration of FK506. Inulin/PAH clearance indicated a decrease in glomerular filtration rate and renal plasma flow with an increase in renal vascular resistance. The renal histology showed vacuolization in proximal tubuli and media of smooth muscle cells of arterioles. The administration of Dilt functionally and morphologically improved renal impairment induced by FK506. In conclusion, FK506 induces a dose dependent decrease in renal function with significant histological changes in tubuli and arterioles in rat kidney. Intracellular calcium deregulation seems to be involved in FK506-induced nephrotoxicity.