The second inward current (i_si) was studied by the two microelectrode voltage clamp technique in the sino-atrial node of the rabbit. The slow component (i_si,2) of the second inward current was sometimes identified and i_si behaved as if it were a mixture of two currents. We analysed the (i_si,2) 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_si was activated. 2) It was shown that there are three categories of the i_si 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_si activation in the peak level and the time course. Thirdly the only slow component of i_si was found in rare cases. 3) The activation of (i_si,2) was dependent upon membrane potential. The i_si shows two separate peaks during clamp depolarizations and higher clamp pulses lead to fusion of the peaks. 4) The i_si,2 activation showed that it decreased with repetitive clamp pulses and it was more evident in higher frequencies(2Hz)(negative staircase).