Atrioventricular (AV) conduction disturbances in patients (pts) with sick sinus syndrome (SSS) are well known, but sinus node function (SNF) in AV block is not well documented. We therefore performed electrophysiological tests to evaluate SNF in 30 pts with high-degree AV block (group 1) and AV conduction in 15 pts with SSS (group 1). These measurements were repeated after vagolysis with atropine in group 2. In group 1 ergometry was performed if the electrophysiological SNF tests were abnormal. Results (mean +/- 1 SD) for group 1 were: sinus node recovery time (SNRT) 1184 +/- 473 ms, corrected SNRT (cSNRT) 337 +/- 394 ms. In 3 pts SNRT or cSNRT was abnormal. In these 3 pts the response of sinus rate to exercise was insufficient. In group 2 SNRT before and after atropine was 2345 +/- 822 ms and 1558 +/- 815 ms respectively (p less than 0.05), cSNRT 1285 +/- 965 ms and 954 +/- 832 ms (n.s.), sinoatrial conduction time 319 +/- 283 ms and 150 +/- 149 ms (n.s.), and Wenckebach point 532 +/- 178 ms and 383 +/- 68 ms (p less than 0.01). His-ventricle time was 48 +/- 5.8 ms. In 6 of these 15 pts impaired AV-conduction was present (defined as Wenckebach point below 500 ms), but normalized in 4 pts after atropine. We postulate that persistence of abnormal SNF and AV conduction after vagolysis is an argument for organic binodal disease. This occurs with equal frequency in both groups in about 10% of patients. Increased vagal tone is common in SSS and uncommon in AV block. In patients with SSS the frequent additional AV conduction delay must be taken into consideration when atrial pacing is considered.