The voltage clamp studies were undertaken to elucidate the properties of the slow inward current, i<sub>si</sub> in the small preparations of the rabbit sinoatrial node. The slow inward current, i<sub>si</sub> which is known to be responsible for the late one-third of pacemaker potential and whole range of upstroke phase of action potential was analysed with the effects of isoprenaline, cobalt, ouabain and higenamine. The results obtained are as follows; 1) Voltage of SA node preparation was held at zero current level, usually-40mV and the slow inward current, i<sub>si</sub> was activated by depolarizing clamp pulses. Peak values of i<sub>si</sub> in steady state were at -10±0mV in most preparations. 2) Isoprenaline, β-agonist increased i<sub>si</sub> and no shift was noticed in voltage-dependency. 3) Cobalt ion in the concentration of 1 mM abolished is, in entire range of membrane potential and the difference of two current levels before and after Co<sup>2+</sup> treatment could be considered as pure i<sub>si</sub> magnitude. 4) In the therapeutic concentration of ouabain (5 X 10<sup>-8</sup>M) slightly increased is, and reduced the time to reach the peak value. 5) Higenamine (10<sup>-6</sup>M) changed the configurations of action potential (i. e. rapid upstroke phase and notch in the spike) and increase spontaneous rate. It also increased is, and the effect of higenamine was blocked β-blocker, propranolol (10<sup>-6</sup>M).