Despite the considerable interest and a big amount of studies on various aspects of the electrophysiology of cells that initiation heart rhythm, the understanding of the mechanisms underlying their electrogenesis is insufficient. The main goal of this work is to analyze the effects of blockers of K +, Na + and Ca2 + channels on the configuration of the phases of the action potential and to determine which of the currents is pivotal. Method. The experiments were performed on spontaneously contracting heart of ascidia Styela rustica (n = 13) using microelectrode technique. Isolated heart was perfused with aerated sea (White See) water (mmol): 280 NaCl; 6 KCl; 36 MgCl2; 7 CaCl2; 19 Na2SO4; pH ~ 8.1 (Kuzmin et al., 2012). Results. Action potentials of the myoepithelial cells in the heart were divided into two types: the action potential with slow diastolic depolarization (SDD; pacemaker) and without diastolic depolarization (working or contractile ). Cells without SDD generated AP with a higher amplitude 96 ± 11 mV, and dV / dtmax 7 ± 1 V / sec (n = 12) comparing to the pacemaker type cells 71 ± 6 mV and 1.6 ± 0.3 V / sec (n = 4). All ascidia heart cells exhibited as episodes with burst from 7-16 action potentials. Then there was a pause for about a minute, after which there was a burst of APs on the other, turned to the stomach, side of the heart. On average, the frequency of action potential generation was 11 ± 4 pulses per min, without counting the time of “silence” or pause. Lidocaine (1 mM) decreased the dV / dt max in the contractile cells from 4.9 ± 0.3 V / sec up to 3.7 ± 0.4 V / sec (n = 5; 21%), and lengthed prolonged the action potential at 20 % of repolarization, APD 20 from 75 to 120 ms compared to the control . In pacemaker cells 4-aminopyridine (4AP; 5 mM) increased APD20 three times and decreased the upstroke velocity dV / dt max by more than 50% compared with the control. The period of the action potential generation in this burst did not changed. The increasing of the amount of APs in bursts, the decrease of the time of pauses in two-three times, the increasing of action potentials duration at 90 % of repolarization by 51% compared to the control were registered after the 30 minutes exposure to EGTA (5 mM). The data indicate that K-currents play a pivotal role in the generation of pacemakering of ascidia’s heart myoepithelial cells.