Sinus bradycardia and total atrioventricular block were found at the age of seven months in a boy who is now 16 1/2 years old. According to Yaters (1929) criteria, these are likely to be congenital disorders. While the frequency of the ventricular impulse generator in the upper bundle of His only slowed in accordance with age during the period of observation, the sinus node activity deteriorated considerably. Finally potentials could only be sporadically demonstrated. Under heavy stress and with a simultaneous slight increase in ventricular frequency, a re-occurence of regular but slow sinus node activity came about. The cause of this binodal disorder of rhythm is unknown. There is no indication of family affliction or of myocarditis in early childhood. Congenital heart disease could also be excluded.