A series of 4,5-dihydro-1-phenyl-1H-2,4-benzodiazepines has been identified as potential antiarrhythmic agents that interact with sodium and potassium channels and prolong the ventricular effective refractory period (ERP) in anesthetized guinea pigs. Concomitant displacement of radiolabeled bactrachotoxin from site II in Na+ channels and of radiolabeled dofetilide from delayed rectifier K+ channels was evident with all members of this chemical series at a concentration of 10 microM. Structure-activity relationship (SAR) studies using a paced guinea pig model to assess prolongation of the ERP indicated that methyl or ethyl at the 1-position had little effect on activity, while larger groups caused a diminution of activity. Compounds with substituents at either the 3- or 4-position that increased lipophilicity generally were more potent; however, too many lipophilic substituents simultaneously at positions 1, 3, and 4 resulted in less active compounds. Substituents on either aromatic ring had little influence on activity, and phenyl at the 5-position resulted in a significant reduction in antiarrhythmic activity. When two sets of enantiomerically pure compounds were tested in the guinea pig, chirality was shown to be important for activity of 8, where the (R)-enantiomer was the more active, but not in the case of 15, where the enantiomers were equiactive. Several compounds in this series increased the threshold for ventricular fibrillation and refractoriness in myocardially-infarcted anesthetized cata and delayed the onset of aconitine-induced arrhythmias in anesthetized guinea pigs following intravenous dosing. Moreover, these compounds possessed oral antiarrhythmic activity in conscious myocardially-infarcted dogs. Compound R-15 has been advanced for further biological and toxicological evaluations.