Enhanced atrioventrcular nodal conduction (EAVC) comprises a portion of the spectrum of atrioventricular node electrophysiology in which physiologic slowing of atrioventricular conduction is diminished. To evaluate the basis of EAVC in humans, we compared the effect of pharmacologic autonomic blockade on electrophysiologic characteristics of atrioventricular nodal conduction and refractoriness in 10 patients with EAVC and 12 patients without EAVC (non-EAVC). The effects of autonomic blockade (0.2 mg/kg iv propranolol and 0.04 mg/kg iv atropine) on sinus cycle length and atrial refractory period were similar in both EAVC and non-EAVC patients. After autonomic blockade, minimum atrial paced cycle length with 1:1 atrioventricular conduction lengthened in EAVC patients (control, 268 ± 25 msec; blockade, 307 ± 34 msec; p < .01), but remained unchanged in non-EAVC patients (control, 392 ± 69 msec; blockade, 382 ± 49 msec; p = NS). Similarly, atrioventricular nodal functional refractory periods tended to lengthen in EAVC patients, while remaining unchanged or shortening in non-EAVC patients. Furthermore, although autonomic blockade reduced differences between EAVC and non-EAVC patients, both minimum cycle length with 1:1 atrioventricular conduction and atrioventricular functional refractory period remained shorter in EAVC than in non-EAVC patients (p < .02). Thus, baseline differences in autonomic atrioventricular nodal control differ in EAVC and non-EAVC patients, but do not fully account for differences in atrioventricular nodal electrophysiologic properties in these patients. Consequently, underlying structural functional factors may play an important role in determining the range of atrioventricular nodal electrophysiologic characteristics in man.