The aim of the present study was to verify the hypothesis about the contribution of the slow component of delayed rectified K-current (IKs) in the mouse sino-auricular node (SA) pacemaker cells. Methods. Experiments were performed on albino male mice (30±5 g) two months of age (n=13). Using microelectrode technique we examined the effects of 4-chromanol on spontaneously beating strips of sino-auricular region. Results. Application of 4-chromanol (30 µm) slowed the final phase of repolarization from 0.59±0.09 V/sec to 0.43±0.09 V/sec (- 27%, n=5; p ≤0.05) in the true pacemaker cells (dV/dt max ≤ 5V/sec). The velocity of the diastolic depolarization decreased from 146 mV/sec to 113 mV/sec. The frequency of action potential (AP) generation in strips decreased from 308±34 beats per min to 260±29 beats per min compared with the control. The 4-chromanol (30 µm) did not change the other parameters of AP (for example, the amplitude, dV/dtmax). This made it possible to consider this drug as a specific blocker of the slow component of delayed rectified K-current (IKs). The frequency of AP generation was 286±30 beats per min in the latent pacemaker cells (dV/dt max ≥10 V/sec) (n strips = 10). Application of IKs inhibitor 4-chromanol (30 µm) slowed the phase of the “plateau” from 0.43 V/sec to 0.25 V/sec (n=6; p≤ 0.05). The phase of final repolarization velocity was decrease from 0.75±0.13 V/sec to 0.37±0.1 V/sec (- 51%). The velocity of the diastolic depolarization was reduced by 52%. As a result, the frequency of AP generation decreased by an average of 31% compared with the control. Interestingly, that 4-chromanol (30 µm) decreased V0max of AP in four times compared with the control and in twice times compared with 10 µm in latent pacemaker cells. Conclusion. The data have indicated that the contribution of the current sensitive to 4-chromanol (presumably current IKs) to repolarization and diastolic depolarization phase cells SA node mouse cells is 20-30%. This work was supported by the Ural Branch of the Russian Academy of Sciences (project 12-P-4-1054, 12-U-4-1022) and Russian Foundation for Basic Research (project 12-04-32288 (M.G.).