We compared the effects of the calcium channel blocking drugs verapamil and nifedipine on atrioventricular (AV) nodal function by His bundle electrocardiography in 24 pigs. AV nodal conduction time was decreased 16% by nifedipine (0.1 mg/kg + 3 micrograms/kg/min; p less than 0.05) and increased 20% by verapamil (0.2 mg/kg + 10 micrograms/kg/min; p less than 0.05). Pretreatment with a beta-adrenoceptor blocking agent (metoprolol, 0.4 mg/kg) prevented the decrease in AV nodal conduction time by nifedipine. Vasodilatation alone (nitroprusside, 20 micrograms/kg/min) produced a 16% decrease in AV nodal conduction time and a 19% decrease in cycle length. Before any of the drugs was given, AV nodal conduction time correlated negatively with the heart rate (rho = -0.65; p less than 0.001; n = 24). beta-Adrenoceptor blockade (metoprolol, 0.4 mg/kg) resulted in a 13% increase in cycle length and an 8% increase in AV nodal conduction time. These results indicate that, even with high concentrations, the effect of nifedipine on the AV node in vivo is primarily a facilitation caused by a baroreceptor reflex. In contrast, verapamil causes a depression of the AV node without such an increase in sympathetic tone.