The effects of 2-[p-(dimethylamino)styryl] pyridine methiodide (DSPM) on rat ECG and HBE were studied. The P-R and R-R intervals of ECG were lengthened 40.3% and 115.4% respectively, after iv injection of DSPM 2 mg/kg. DSPM (1.5 and 2 mg/kg) affected the A-H interval in a dose-dependent manner with no change in the H-V interval of HBE. DSPM 0.5 mumol/L lengthened the A-H interval of isolated perfused hearts in frequency-dependent manner, with greater prolongation at higher frequency. The effect of verapamil on the A-H interval was less than that of DSPM. Propranolol had no effect on the frequency-dependent delay of atrioventricular node. DSPM 0.5 mumol/L lengthened the functional and effective refractory periods of atrioventricular node by 26.8% and 31% respectively. These phenomena support that DSPM may be a calcium antagonist.