The second inward current (i<sub>si</sub>) was studied by the two microelectrode voltage clamp technique in the sino-atrial node of the rabbit. The slow component (i<sub>si,2</sub>) of the second inward current was sometimes identified and i<sub>si</sub> behaved as if it were a mixture of two currents. We analysed the (i<sub>si,2</sub>) in relation to membrane potential and frequency of voltage clamp pulses. 1) Membrane was held at -40mV which was usually found to be zero current level. When depolarizing pulses were applied, the slow inward current i<sub>si</sub> was activated. 2) It was shown that there are three categories of the i<sub>si</sub> activation by the low level of depolarizing clamp pulses. Moderately fast inward current with single component was developed in most cases in the presence of tetrodotoxin(TTX). But sometimes there was two separate components of i<sub>si</sub> activation in the peak level and the time course. Thirdly the only slow component of i<sub>si</sub> was found in rare cases. 3) The activation of (i<sub>si,2</sub>) was dependent upon membrane potential. The i<sub>si</sub> shows two separate peaks during clamp depolarizations and higher clamp pulses lead to fusion of the peaks. 4) The i<sub>si,2</sub> activation showed that it decreased with repetitive clamp pulses and it was more evident in higher frequencies(2Hz)(negative staircase).