The effects of disopyramide (DP) and a new antiarrhythmic agent, disobutamide (DB) on cardiac conduction were studied using His bundle recording from modified rabbit Langendorff preparations electrically driven at 3 and 4 Hz. Both disopyramide (4-16 microgram/mL) and disobutamide (1-30 microgram/ml) showed conduction throughout the atrioventricular conduction system, i.e., SA, AH, and HV intervals were increased in a dose-related manner. Conversion of the conduction time changes to percent changes indicates that disobutamide had a relatively equal effect on each part of the system whereas disopyramide exhibited significantly less effect on AV nodal conduction. Slowing of conduction in the AV node by DP was clearly related to rate. Changes in SA and HV intervals were rate related to a lesser degree. No such rate-related effect was evident with disobutamide. Block of arterial conduction occurred in two out of six hearts when the rate was increased 8 microgram/mL of DP and in three additional hearts at 16 microgram/mL. This was interpreted to indicate a change in atrial excitability such that 2 X threshold currents no longer excited the tissues. This was not observed at any concentration of DB.