The normal period of depression of sinoatrial node automaticity (sinus node recovery time, SNRT) following one minute of overdrive right atrial pacing was evaluated in 34 subjects, aged 27--83 years. Specific attention was paid to the influence of the patient's heart rate, atrial pacing rate (100 and 130 bpm) and site, and of vagal and sympathetic effects, as assessed by observations following the administration of atropine, 0 . 03 mg/kg, and propranolol, 0 . 15 mg/kg. Normal SNRT was 1046 +/- 17 ms at 100 bpm and 980 +/- 19 ms at 130 bpm. Linear regression analysis showed that at pacing rates of both 100 and 130 bpm, both before and after autonomic block, a highly significant relation existed between SNRT and the stable P-P interval observed after cessation of pacing. These regression equations were used to develop a correction factor for cycle length in assessing SNRT (corrected SNRT = SNRT--0 . 86 X cycle length, where 0 . 86 was the slope of the regression equation). The mean corrected SNRT was 314 +/- 10 ms and 290 +/- 8 ms at 100 and 130 bpm, respectively. Vagal influences increased SNRT and were of greater magnitude than the decrease in SNRT due to sympathetic effects. Corrected SNRT was significantly longer following left atrial than following right atrial pacing, but in those eight patients studied, was not significantly different following right atrial or right ventricular stimulation.