The main determinant of conduction velocity is the AP maximum depolarization rate (dEm/dtmax), which depends on the voltage-gated Na+ channels biophysical properties. Here we investigated age-dependent differences in whole-cell Na+ current (INa) and dEm/dtmax in myocytes isolated from neonatal and adult Wistar rats. The impact of Na+ channel biophysical properties on AP overshoot was evaluated with an AP model developed with measured whole-cell INa, Ca2+, and transient outward and delayed rectifier K+ currents from immature cells, described according to Hodgkin-Huxley kinetics. Even though INa density was 2-fold greater in neonatal myocytes (−71.9 ± 35.5, n = 11; 33.3 ± 1 6.7 pA/pF in adults, n = 6; p < .01), dEm/dtmax was not, and tended to be lower in neonates (84.2 ± 35.8 V/s, n = 12) than in adults (100.3 ± 44.4 V/s, n = 7; p = .40). The half-maximal activation voltage (E1/2) was less negative (−31.5 ± 1.7 mV vs. -44.4 ± 6.0 mV; p < .001), and the activation curve had greater slope (k: 7.8 ± 1.2 mV vs. 4.4 ± 0.6 mV; p < .001) in immature than in adult cells. A positive shift was also observed in the steady-state inactivation curve in neonates (E1/2: −76.2 ± 7.8 mV vs. -85.6 ± 4.1 mV; p < .05). The dEm/dtmax of the simulated AP in neonatal myocyte was 81 V/s. However, when the activation and inactivation equations were adjusted to reproduce E1/2 and k values estimated in adult cells, dEm/dtmax increased to 96 V/s, close to the experimental value in adults. Therefore, it seems that, in addition to INa density, voltage-dependence of Na+ channel activation and inactivation may exert marked influence on depolarization rate, and AP propagation velocity.