His-bundle electrocardiography was used to evaluate the effects of ethmozine on cardiac conduction in isolated perfused rabbit hearts electrically driven at cycle lengths of 320 and 250 ms. There was no significant change in conduction until high concentrations of ethmozine were reached. His-Purkinje and atrioventricular (AV) nodal conduction were slowed significantly at 0.1 microgram/mL and atrial conduction at 1.0 microgram/mL. Conduction block occurred at 10.0 micrograms/mL in all the hearts treated. Effects of the drug (0.1 and 0.01 microgram/mL) on conduction of extrasystoles were also studied in hearts driven at a basic cycle length of 270 ms. No significant change was observed in atrial conduction of extrasystoles throughout the coupling intervals tested at both concentrations. Ethmozine (0.01 and 0.1 microgram/mL) caused slowing of His-Purkinje conduction of extrasystoles but the effect of the drug did not change as a function of the coupling interval. An interval-dependent increase in AV-nodal conduction time was observed, with the maximum slowing of conduction occurring at coupling intervals close to the effective refractory period of the AV node. AV-nodal functional refractory period was increased significantly by ethmozine (0.01 and 0.1 microgram/mL). The effective refractory period was significantly increased only at the higher concentration.