SummaryPresent knowledge on the electrical activity of the myocardial cell and the transmembrane ionic fluxes is summarized.. The effects of experimental variations of plasma levels of K, Na and Ca on the activity of digitalis glucosides arc recalled.Clinical illustrations of the interaction between digitalis and 1) hypokalæmia, 2) hyperkalæmia, 3) hypernatræmia are presented.Abolition of glucoside toxictiy by K salts, even at normal plasma K concentration, is illustrated.