This study examined the complex interaction between vagal enhancement and how a concealed atrial impulse alters atrioventricular (AV) nodal function. In theory, vagal augmentation could increase or decrease the effect that a premature atrial beat has on the subsequent beat. In 10 patients we established the AV nodal effective refractory period (ERP) without and with a conditioning atrial stimulus (Sc); the stimulation protocol was then repeated after enhancing reflex vagal tone with a continuous phenylephrine infusion. During phenylephrine infusion, the sinus cycle length prolonged from 827 +/- 99 to 1,029 +/- 223 ms (p < 0.001) and AV nodal ERP increased from 331 +/- 51 to 425 +/- 64 ms (p < 0.005). At control, AV nodal ERP in the presence of Sc prolonged to 536 +/- 69 ms (p < 0.001), and during phenylephrine infusion increased to 579 +/- 57 ms (p < 0.01), a change significantly less than during control (58 +/- 14% vs 31 +/- 14%, respectively, p < 0.01). Further experiments suggest that the effect of Sc was reduced because it occurred earlier relative to the vagally prolonged AV nodal ERP. In conclusion, this study demonstrates a complex relation between the timing of a premature atrial beat causing concealed conduction and the degree of vagal tone. The concealed beat, as related to the AV node ERP, has a substantial effect on subsequent AV nodal conduction. These data give insights into clinical AV nodal function.