We have examined the effects of trifluperazine and fluphenazine on action potentials and transient depolarizations of rabbit ventricular cells. Isolated myocytes were prepared by perfusing rabbit hearts with low calcium enzyme-containing solutions, and action potentials were stimulated at 1 Hz and recorded using patch-type pipettes. In normal saline, 10 microM trifluperazine or fluphenazine shifted the action potential plateau to more negative potentials and increased the rate of phase 2 repolarization. Transient diastolic depolarizations appeared in solutions containing 50 nM isoproterenol plus 1 microM strophanthidin. These transient depolarizations were abolished by the addition of 10 microM trifluperazine or fluphenazine. In addition, spontaneous transient depolarizations were occasionally observed, and these too were abolished by the phenothiazines. Because these compounds are potent inhibitors of calmodulin, these data raise the possibility that calcium-calmodulin-regulated processes are important in the generation of arrhythmogenic transient depolarizations.