Abstract: Effects of Na₂Seo₃ on the spontaneous electric activity in guinea pig ventricular muscle induced by BaCl₂ and on the rabbit sinoatrial node were observed using glass microelectrode technique. The results showed that addition of 0.6mM Na₂Seo₃ caused the barium-induced spontaneous electric activity in ventricular muscle to decrease or disappear. When sinoatrial node cells were exposed to 0.9mM Na₂Seo₃, their APA, SPo and SP₄ decreased and APD₉₀, SCL were prolonged. While, when the concentration of Na₂Seo₃ was increased to 1.2raM, the AP of sinoatrial node cells disappeared. The results indicate that Na₂Seo₃ may suppress the spontaneous electric activity.