Nine patients with testicular cancer were treated with four platinum containing chemotherapy courses. In order to prevent cisplatin (CDDP)-induced nephrotoxicity, verapamil, a calcium entry blocker, was added to the chemotherapeutic regimen. Renal function studies, consisting of the measurement of the effective renal plasma flow (ERPF) and the glomerular filtration rate (GFR), were performed before, during and after the first dose of CDDP, at the end of the first course and at the end of the fourth course of chemotherapy. The results were compared with the results obtained previously in CDDP-treated patients not receiving verapamil. The initial decrease in ERPF during and after the first CDDP infusion was completely prevented by verapamil. Despite continuation of verapamil throughout all four courses, it failed to reduce the ultimate fall in GFR.