Nephrotoxicity is a serious side effect that limits the use of cyclosporine (CyA) as an immunosuppressive agent. The purpose of this study was to develop a model of CyA nephrotoxicity in the isolated perfused rat kidney and to evaluate the effects of adenosine triphosphate (ATP)-MgCl2 and verapamil treatment on this model. Kidneys were perfused for 90 minutes in a 7.5% albumin-Krebs-HCO3 solution containing 3H-inulin (glomerular marker) and 5.0 μg/ml 14C-cytochrome C (cyt) (marker of tubular protein absorption). After 10-minute equilibration, perfusate and urine samples were collected with 10-minute clearance periods. After two control clearance periods, 500 ng/ml CyA was added to the perfusate. In some experiments, 1 μg/ml of verapamil was added 10 minutes before CyA and in others 2 mmol/L ATP-MgCL2 was added with CyA. Cyt and inulin radioactivity, [Na+] and [K+], were measured in perfusate and urine. Tissue ATP levels were also determined. The results demonstrate that CyA treatment leads to a marked depression of glomerular filtration rate (GFR), tubular absorption of protein, urine output, and renal flow. ATP-MgCl2 cotreatment improved GFR, tubular absorption, and renal perfusate flow but the increase in urine output was not dramatic. Verapamil pretreatment markedly improved GFR and urine and renal perfusate flow but not tubular function. The combination of verapamil and ATP-MgCl2 treatment with CyA returned GFR to control values, significantly improved tubular absorption, urine and renal perfusate flow, and enhanced renal tissue ATP of isolated kidneys to levels seen in vivo. These data lead us to conclude that ATP-MgCl2 cotreatment with CyA after verapamil pretreatment greatly reduces the nephrotoxic potential of this immunosuppressive agent.