Duck hearts (embryonic) were observed for the occurrence of either auriculoventricular block or missing beats when placed in Tyrode''s soln. containing various amts. of digitalis glycoside (Lanatoside C). The time of occurrence of arrhythmia varied inversely with the concentration of glycoside used. No fixed latent period existed between the time of contact of the heart with glycoside and the occurrence of arrhythmia, for if sufficient glycoside is present, arrhythmia may occur almost immediately.