The transient outward current (I to) has been identified in sinoatrial node of rabbit. However neither its physiological role has yet been clarified (Uese et al.1999; Boyett et al., 2001; Verkerk, van Ginneken, 2001). Very little is known about of the 4-aminopyridine (4-AP) sensitive current in sinoatrial (SA) node of the mouse (Lavrik, Golovko 2007; Kharche et al. 2011). The goals of the present study is to investigate the characteristics action potential (AP) and I to possible contribution to the action potential (AP) generation in the male albino mouse (two-month-old; Mus domesticus; n strips = 34) spontaneously contractions strips using the microelectrode method. The mean frequency of AP generation was 312 ± 29 beats per min, bpm (K+ 5.4 mM; Ca2+ 1.8 mM ; 31 C). Aminopyridine (0.5 mM, “Sigma”) lengthened the action potential duration at the 20% repolarization (APD20) by 50% in pacemaker cells with upstroke velocity dV/dtmax – 3±1 V/s (mean ± SE, n cells =19) was registered. This was accompanied by decreasing of dV/dtmax 30-50% compared with the control (n=5; p < 0.01; fig. 1). It has been also identified that 4-AP does not cause lengthened of the final repolarization phase (phase 3). Action potential auricle type have been recorded also from subendocardial side SA area and characterized short APD20 – 7±2 ms (n cells =10). 4 -AP (0.1 mM) lengthened the APD20 (really plateau) by 50% (EC50) in auricle cells with dV/dt max – 110±6 V/s was registered. In other words we found that shorter spike AP than was higher 4-AP sensitivity SA area mouse cells. The received results don’t explain the phenomenon of decrease upstroke velocity, which was first time recorded at exposure 4-AP. However some assumptions can be made about the transition from phase 0 to phase 2 (“plateau”) at exposure 4-AP. Block of I to current increased [K+]i , which indirectly slowed the conductance of Na channels – seems to us most likely. Supported by the Ural’s Division RAS 11-4-HP-438, 12-P-4-1054, 12-U-4-1022.