In 30 symptomatic patients with complete atrioventricular block, sinus node recovery time was measured in order to establish the incidence of sinus node dysfunction. Prolonged corrected sinus node recovery time ( > 560 msec) was found in seven cases (23%). Sinus node recovery time expressed as percentage change of the basic cycle length exceeded the upper limit of normal ( > 170%) in nine cases (30%). The mean value of these parameters was 563 msec and 179%, respectively. The study shows that sinus node dysfunction frequently coexists with complete atrioventricular block.