Abstract: Change of myocardial cell action potential(AP) and the effects of Na₂SeO₃ on the change were studied on animal models of experimental coronary artery spasm in guinea pigs with flexibly mounted ultramicroelectrode technique. After pituitrin was given intravenously(0.3U/kg), APH was lowered by 28%, and APD50 and APD₉₀ were shortened by 17% and 15% respectively. The AP did not show any obvious change following intravenous injection of Na₂SeO₃(1 mg/kg). However, on the basis of the injection of Na₂SeO₃, the same dosage of pituitrin induced much less effect than injection of pituitrin only. The results suggest that Na₂SeO₃ has certain antagonistic action on the changes of AP induced by experimental coronary artery spasm.