The hemodynamic effect of methylepoxyproscillaridin (3'-methyl-4'-5'-epoxy-proscillaridin) (P35) was studied according to the Swan Ganz thermodilution method in 6 patients with latent cardiac insufficiency. Both at rest and under ergometer exercise, the stroke volume and the systolic blood pressures increased under P35. Moreover, under physical exercise the heart rate decreased significantly, while cardiac output increased. The hemodynamic effect of P35 can therefore be regarded as typical of cardiac glycosides.