The spontaneous activity of pacemaker cells in the sino-atrial node (SAN) controls the heart rhythm and rate (HR) under physiological conditions. Automaticity is due to the diastolic depolarization (DD), a slow depolarization phase which drives the membrane voltage from the end of an action potential to the threshold of a new action potential. SAN cells express a wide array of ionic channels, but we have limited knowledge about their functional role in the genesis and regulation of heart automaticity. Particularly, the role of L-and T-type calcium channels in the generation of the DD has been matter of debate. Indeed, even if L- and T-type channels have been proposed to contribute to pacemaking (1), we lack genetic evidence linking the activity of specific voltage-dependent calcium channels genes to dysfunction of cardiac automaticity. We have thus developed a technique to isolate pacemaker cells from mouse SAN (2) and AVN (3). During the last few years, we have studied pacemaker activity in mice lacking L-type Cav1.3 (Cav1.3-/-)(4) and T-type Cav3.1 (Cav3.1-/-) calcium channels (3). We found severely slowed and erratic pacemaking in SAN and AVN cells from Cav1.3-/- mice and demonstrated that this phenomenon is due to abolition of ICa,L in the diastolic depolarization range. Cav1.3-/- mice have pronounced bardycardia, strong SAN dysrhythmia and display sporadic II-degree AV blocks (4). Atrial fibrillation and flutter were also observed in Cav1.3-/- mice. SAN dysrhythmia, but not bradycardia could be compensated by crossing Cav1.3-/- animals with mice lacking the cardiac IKAch (5). This observation indicates that Cav1.3 channels are essential for stabilizing SAN rate under vagal activation. On the other hand, disruption of the gene coding for Cav3.1 channels abolished ICa,T in SAN and AVN cells. Cav3.1-/- mice had moderately reduced HR (10%) and slowed AVN conduction (3). The lack of Cav3.1 channels prolonged the SAN recovery time and slowed pacemaker activity of individual SAN cells through a pure reduction of the slope of the diastolic depolarisation. However, the heart rate variability (HRV)of Cav3.1-/- mice was not significantly different from that of wild-type animals, indicating that even if Cav3.1 channels contribute to the setting of the basal HR they have reduced impact on the autonomic regulation of HR. In conclusion, our studies indicate that L-type Cav1.3 and T-type Cav3.1 channels play specific and differential roles in the genesis and pacemaker activity. A model of activation of these channels during mouse SAN pacemaking will be discussed.