Electrophysiological difference of the central and peripheral area of the sinoatrial node in the rabbit was studied by glass microelectrode technique. Effects of K⁺, NaK⁺, CsK⁺, adrenaline and ouabain on the action potential of the two areas were investigated, and transient hyperpolarization (K⁺-induced hyperpolarization) which developed following readmission of potassium after having pre-treated with K⁺-free Tyrode solution for 10 minutes was analyzed. The results obtained were as follows ; 1) The frequency of the spontaneous action potential recorded in the periphery of the SA node was faster than the central area. Reduction by Cs⁺ and increase by O mM K⁺, 10^-6 M adrenaline and 10^-6M ouabain in the frequency of action potential were noticed more prominently in the peripheral than the central area. On the contrary, the frequency in the central area was more decreased than the Peripheral area by 13 mH K⁺ and 1 mM Co²⁺. 2) The amplitude of the K+_induced hyperpolarization was very small in the central area but large in the peripheral area. Transient hyperpolarization was abolished by ouabain and low sodium, and decreased by cooling the tissue (17℃). 3) By changing the concentration of Ca²⁺ in the perfusate, the amplitude and the rate of transient hyperpolarization were increased in the high Ca²⁺ concentration. It could be concluded that the central area of the SA node is less susceptible to the inhibition of Na-Pump and more susceptible to Ca-blocker and high concentration of K⁺. The Na-Pump activity of the central area measured by means of transient hyperpolarization is found to be much less active than that of the peripheral area.