Dihydropyridines (DHP) have great potential for clinical use because of their inotropic and vasomotor effects. The mechanism of action is unknown although Ca currents have been implicated. Here we report measurements of single channel and whole cell cardiac Ca currents after application of two DHP agonists BAY K 8644 and CGP 28 392. Whole cell Ca currents from individual myocytes were increased and the 50% effective doses (ED50) were similar to those reported for contractility in rabbit aorta and guinea pig heart and catecholamine release from cat adrenal glands. The measured ED50 was also consistent with the apparent dissociation constant (Kd) of a high affinity binding site present in cardiac sarcolemmal vesicles. We propose that the molecular basis for these results is an increase in the probability that a single Ca channel, having opened and closed, will subsequently re-open during membrane depolarization. At high concentrations of BAY K 8644 and in the presence of 96 mM Ba, different effects are observed, primarily a marked prologation of open time.