Transient inward current (TI) was studied by the two micro-electrode voltage clamp technique in the sinoatrial node of the rabbit. The author confirmed that in 10<sup>-6</sup> M ouabain TI was found in the SA node and investigated the effects of ions, (Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, β-agonist (isoprenaline), local anesthetics (quinidine, lidocaine) and Ca-blockers (Co<sup>2+</sup>, verapamil, diltiazem) on the TI recorded during depolarizing voltage clamp pulses to -40 and -20 mV. The results obtained were as follows ; 1) 10<sup>-6</sup> M ouabain increased the frequency of sinus action potential and decreased the amplitude, especially overshoot of action potential. TI was induced by the depolarizing voltage clamp Pulses and the magnitude of the slow inward current (isi) decreased and the time course was slowed by the same depolarizing pulses. 2) 30% Na<sup>+</sup> and 24mM K<sup>+</sup> decreased by 10<sup>-6</sup> M ouabain and 6 mM Ca<sup>2+</sup> and 10<sup>-7</sup> M isoprenaline increased TI, i<sub>si</sub> and current oscillations. 3) Quinidine 5 X 10<sup>-7</sup>M) reduced TI and i<sub>si</sub> but lidocaine (10<sup>6</sup> -10<sup>-5</sup> M) didn t reduced or increase TI. Current oscillations increased and isi decreased by lidocaine. 4) Ca-blockers decreased the amplitude and the frequency of sinus action potential. TI and i<sub>si</sub> decreased significantly but were not abolished completely at the concentrations used in this experiment. Verapamil and diltiazem had inhibitory action on TI in 2 X 10<sup>-7</sup>M concentration and showed very slow recovery after wasting out with normal Tyrode solution.