Sematilide (N-[2-(diethylamino)ethyl]-4- [(methylsulfonyl)amino]benzamide HCl) is a new class III antiarrhythmic agent that has been shown to be effective in preventing reentrant ventricular arrhythmias in experimental animals and humans. In this study, we examined the in vitro effects of sematilide (1-100 microM) on isolated sinoatrial (SA) node, atrioventricular (AV) node, and atrial muscle. These results were then compared to another class III agent, clofilium (1-30 microM). In SA nodal tissue, sematilide increased the action potential duration (APD) and spontaneous cycle length (SCL) in a concentration-dependent manner (EC20% = 15 +/- 3 and 54 +/- 13 microM, respectively). In addition, there was a slight reduction in maximum diastolic potential at 100 microM. Clofilium had similar class III effects, but was approximately 3 to 18 times more potent (EC20% = 6 +/- 2 and 3 +/- 1 microM for the APD and SCL, respectively). Neither agent had a significant effect on the slope of phase 4 nor on other action potential parameters. Results in AV nodal preparations were similar. Both sematilide and clofilium increased the APD and SCL in a concentration-dependent manner, with clofilium being approximately four to six times more potent than sematilide (EC20% for the APD and SCL for sematilide = 12 +/- 4 and 12 +/- 8 microM, respectively, and for clofilium = 2 +/- 1 and 3 +/- 2 microM, respectively). No significant effects were observed on other action potential parameters. Sematilide and clofilium increased the APD and effective refractory period (ERP) in atrial trabeculae in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)