The effects of 2[p-(dimethyl-amino)styryl]pyridine methiodide (DSPM) on the action potential of dominant pacemaker cells in sinoatrial nodes of rabbits were studied by microelectrode technique. DSPM 45 mumol/L decreased the slope of phase 4, the maximum rate of rise of the action potential and the action potential amplitude by 54% (P less than 0.01), 27% (P less than 0.05) and 13% (P less than 0.05), respectively, and increased the spontaneous sinus cycle length and action potential duration at 90% repolarization by 62 and 26% (P less than 0.01), respectively. These effects of DSPM were similar to that of nifedipine 3.9 mumol/L. The positive chronotropic effect of nicotinamide 33 mmol/L was completedly blocked by DSPM 45 mumol/L while that of isoprenaline 0.11 mumol/L was partially blocked by DSPM 45 mumol/L. The results suggest that DSPM may be a Ca2+ antagonist.