The effects of arenobufagin, a bufadienolide isolated from toad venom, on the Na + /K + pump were investigated by recording the whole-cell current of dissociated cardiac myocytes. Arenobufagin blocked the Na + /K + pump current in a dose-dependent manner with a half-maximal concentration of 0.29 μM and a Hill coefficient of 1.1. Arenobufagin is 3.5-fold more potent than ouabain.