The effects of diltiazem, verapamil, bepridil, nicardipine and nifedipine were studied in vitro on Na+,K(+)-ATPase from dog kidney (EC 3.6.1.37). Except diltiazem, all the drugs tested showed an inhibitory effect on Na+,K(+)-ATPase activity in a dose-dependent manner. Among these drugs bepridil is far more effective than the others (IC50 approximately 10(-4) M). Competition studies showed that bepridil acted in a non-competitive manner with the ATP-Mg2+ complex and in a partially competitive manner with K+. Since ouabain acted similarly under the same experimental conditions, we tested the interaction of bepridil and ouabain on Na+,K(+)-ATPase. With low doses of ouabain, the enzyme inhibition corresponded to a potentiated synergy of the two drugs. We then studied the action of bepridil on the sodium pump activity of intact red blood cells by an ex vivo microcalorimetric technique. At 10(-5) M bepridil caused a significant decrease in sodium pump activity (33 +/- 8%).