Electrophysiologic studies were performed in 10 normals and 33 patients with sick sinus syndrome before and after total autonomic blockade with propranolol and atropine. In normals both corrected sinus node recovery time (SNRT) and sinoatrial conduction time (SACT) decreased significantly after autonomic blockade. In patients with sick sinus syndrome the corrected SNRT was abnormal (greater than 450 msec) in 16 (48.5%) cases before and 25 (76%) cases (greater than 285 msec) after autonomic ablation (P less than 0.02). Thirteen of 21 patients (62%) with normal intrinsic heart rate and all 12 cases with abnormally low intrinsic rate after autonomic blockade had abnormal corrected SNRT (greater than 285 msec). Mean SACT measured in 19 patients also shortened significantly following pharmacologic denervation. During control it was prolonged (greater than 226 msec) in 8 patients (44%). After autonomic blockade 2 of 13 patients with normal intrinsic heart rate and 3 of 6 with low intrinsic rate showed abnormal SACT (greater than 151 msec). The data suggest that the majority (76%) of patients with sick sinus syndrome have intrinsic abnormality of sinus node automaticity while in a minority (24%) disturbed autonomic regulation is the pathogenetic mechanism. Patients with normal intrinsic heart rate usually have normal intrinsic SACT, while a significant proportion of those with low intrinsic rate have abnormal perinodal conduction. Subjects with abnormal intrinsic heart rate have more severe disturbances of sinus node function than those with normal intrinsic rate.