The effect of thevetoidin in heart failure has been studied. Thevetoidin has a prompt and partly transient digitails-like effect which is shown on the heart rate, the circulatory velocity and on the P-R and Q-T intervals of the electrocardiogram, and also a cumulative effect similar to that of strophanthin. Due to its intense vagal effect and its quick digitails-like action, paroxysmal auricular tachycardia was stopped in two patients out of three in from three to four minutes. Its therapeutic advantages and disadvantages are discussed.