Abstract: Standard microelectrode and two-microelectrodes voltage clamp techniques were used to study the effects of linesinine(Lien), an alkaloid extracted from the green seed embryo of Nelumbo nucifera Gaertn, on slow action potentials (AP) and slow inward current (I_si). Lien 10～ 100 μmol/L was shown to concentration-dependently decrease the action potential amplitude (APA), the maximal velocity of phase O depolarization (V_max) and prolong the sinus cycle length (SCL) of slow AP in isolated sinoatrial node (SAN) pacemaker cells of rabbits. On both slow APs of rabbit SAN cells and guinea pig papillary muscles partly depolarized by high K⁺, Bay K 8644 0.03 μmol/L markedly increased the APA and V_max, which were obviously antagonized by Lien 10 μmol/L. Moreover, Lien 1～100 μmol/L was found to inhibit the I_si of canine cardiac Purkinje fiber in concentration—dependent manner. Lien 3 and 100 μmol/L reduced the peak value of I_si by 14% and 88% respectively. The results suggest that Lien possesses calcium antagonistic effects.