To determine whether nifedipine or diltiazem affect digoxin kinetics, glycoside plasma concentrations and renal excretion were measured before and during dosing in 23 patients with cardiac insufficiency achieving steady-state conditions. Mean (+/- SD) digoxin plasma concentration was 0.64 +/- 0.22 before and 0.61 +/- 0.21 ng/ml during nifedipine dosing in 11 subjects over a period of 10 to 14 days. Renal digoxin clearance was not influenced by nifedipine, whereas total body clearance and extrarenal clearance of digoxin increased slightly. In contrast, diltiazem resulted in a 24% to 70% (means = 46%) increase in plasma digoxin concentrations in eight of 12 subjects. Renal digoxin clearance was not influenced by diltiazem, whereas total body clearance and extrarenal clearance of digoxin were reduced 28% and 44% in five of the eight subjects in whom renal digoxin excretion was measured. From these data it was concluded that nifedipine has no significant effects on digoxin kinetics, but that digoxin plasma concentrations should be controlled in subjects receiving digoxin with diltiazem until new steady-state digoxin concentrations are established, and that the digoxin dose be reduced if there is evidence of toxicity.