Abstract: The electrophysiological effects of PTS in sheep cardiac Purkinje fibers were studied. PTS was shown to increase the duration of action potential (APD₃₀, APD₅₀ and APD₉₀) at the concentrations of 2. 5μg/ml and 5. 0μg/ml. However, the amplitude of action potential (APA) remained unchanged. The result of using double microelectrode voltage clamp method showed that PTS (1. 25～10. 0μg/ml) depressed the delayed (outward) rectifier current (Ⅰ_x) in time-and dosedependent manners, when the holding potential was held at±20 mV, the command potential was held at±10 mV, 0.2 Hz and the clamping time at 1～1.5s.It may be concluded that the effect of PTS on APD is mainly related to blocking the delayed rectifier potassium channel.