The electrophysiological properties of sinoatrial (SA) node pacemaker cells vary in different regions of the node. In this study, we have investigated variation of the 4-aminopyridine (4-AP)-sensitive current as a function of the size (as measured by the cell capacitance) of SA node cells to elucidate the ionic mechanisms. The 10 mM 4-AP-sensitive current recorded from rabbit SA node cells was composed of transient and sustained components (Itrans and Isus, respectively). The activation and inactivation properties [activation: membrane potential at which conductance is half-maximally activated (Vh) = 19.3 mV, slope factor (k) = 15.0 mV; inactivation: Vh = -31.5 mV, k = 7.2 mV] as well as the density of Itrans (9.0 pA/pF on average at +50 mV) were independent of cell capacitance. In contrast, the density of Isus (0.97 pA/pF on average at +50 mV) was greater in larger cells, giving rise to a significant correlation with cell capacitance. The greater density of Isus in larger cells (presumably from the periphery) can explain the shorter action potential in the periphery of the SA node compared with that in the center. Thus variation of the 4-AP-sensitive current may be involved in regional differences in repolarization within the SA node.