4-aminopyridine (4-AP)-sensitive current, ITO, participates in the generation of action potentials (AP) in the right atrium cells of the murine heart; however, its functional role is unclear. The aim of our study was to investigate the influence of 4-aminopyridine (4-AP) on main AP parameters of cells in sinoatrial (SA) area of the murine heart. We also analyzed the localization of the pacemaker cells in the right atrium and the dependence of AP parameters on the temperature. Experiments were carried out on the spontaneously beating right atrial strips isolated from hearts of 2-month-old male albino mice (n=10), using the standard microelectrode technique. Data are shown mean± SD. The mouse sinoatrial node is located parallel to the crista terminalis and is separated from the surrounding atrial muscle by an atrial septum. Very little is known about the main parameters of AP in SA area of the murine heart. The SA area has heterogeneous electrophysiological characteristics; therefore, we have divided all registered AP into four types: 1. Dominant pacemaker; 2. Latent pacemaker; 3. Atrial-like AP without phase 1 and without slow diastolic depolarization; 4. Atrial-like AP with phase 1 and without slow diastolic depolarization. The mean frequency of AP generation was 201±18 and varied from 55 to 280 min-1 under control conditions (K+ 5.4 mM, Ca2+ 1.8 mM, 31 °C). The decrease in the perfusion solution temperature from 32 to 24 °C was accompanied by an increase in the duration of the AP at 20% and 90% of repolarization (APD20, APD90) and in the cycle length (CL), according to the exponential dependence (r2 = 0, 88±0,4). The values of Q10 coefficient for APD20, APD90 and CL were 2.5–3.0 at the 32–23 °C temperature range. The cessation of AP generation by the pacemaker cells was registered at the temperature 17±0.5 °C. The supplementation of the perfusion solution with 4-AP (1.6 mM, “Sigma”) led to decrease in the AP amplitude and an increase of the following parameters: APD20 in 3 times, APD50 – 2 times, APD90 – 1.5 times, and CL – nearly 4 times, as compared to the control. Seven minutes after the 4-AP exposure, the AP generation in the atrial-like cells with phase 1 was blocked, and their contractions were completely discontinued. This effect was reversible by 5-minute reperfusion. Additionally, we found that the AP amplitude was greater in the latent pacemaker cells than in the true pacemaker cells. 4-aminopiridine (1.6 mM) blocked the AP generation in the atrial cells and the contraction activity of the sinoatrial area strips.