The effects of 2[p-(dimethylamino)styryl] pyridine methiodide on mouse and rabbit ECG and on the contraction of isolated rabbit atrial muscles were studied. DSPM produced long-lasting bradycardia and A-V block in a dose-dependent manner, but did not effect intraventricular conduction. DSPM antagonized the positive chronotropic and dromotropic effects of isoprenaline (Iso) in vivo, and antagonized the inotropic effects of Iso and CaCl2 non-competitively in vitro. The pD'2 were 4.49 and 4.52, respectively. It is suggested that DSPM may be a Ca2+ antagonist.