Overcoming resistance to chemotherapy is an important goal in cancer treatment. In many systems, resistance to anthracyclines and vinca alkaloids correlates with a diminished intracellular content of drug. In P388 leukemia and Ehrlich ascites tumor, an active outward transport of anthracyclines and vinca alkaloids occurs. Calcium channel blockers, such as verapamil, inhibit this active outward transport and increase intracellular content of vinblastine and anthracyclines in cells resistant to vinca alkaloids and anthracyclines, respectively. We report a phase I trial of vinblastine (1.5 mg/m2 daily as iv continuous infusion X 5 days) in 17 patients and concurrent verapamil in escalating doses. Verapamil was administered as a loading dose (0.02-0.1 mg/kg) followed by a maintenance infusion (0.036-0.18 mg/kg/hour) for 5 1/2 days with continuous cardiac monitoring. There was no apparent augmentation of vinblastine toxicity when vinblastine and verapamil were given concurrently. ECG change was the dose-limiting toxicity. At 0.12 mg/kg/hour, five of nine patients developed first-degree heart block (mean P-R interval, 0.32 seconds; range, 0.23-0.52 seconds). Junctional rhythms were noted in two of 17 patients. Reversible nonspecific T-wave changes were seen in four of 17 patients. Blood pressure and left ventricular ejection fractions (ultrasonic) were not altered. Five of 17 patients had wbc count nadirs less than 2000/mm3, and two of 17 patients had platelet count nadirs less than 100,000/mm3. Four patients experienced neurotoxicity. A mean vinblastine concentration of 2.2 ng/ml (0.55 nM) and a mean verapamil concentration of 290 ng/ml (0.45 microM) were achieved with the concurrent 5-day infusion. The tolerable levels of verapamil obtained appear to be less than those which were reported to inhibit vinblastine efflux in vitro. Additional in vitro experiments at the tolerable doses of vinblastine and verapamil are recommended.