1. The post-natal development of the electrophysiological properties of cortical layer V pyramidal neurons was investigated with intracellular recordings from rat sensorimotor cortical slices, in vitro. 2. At all ages post-natally (post-natal day 1 to day 36; P1-P36) neurons were capable of generating a train of Na+-dependent action potentials in response to intracellular injection of sufficient depolarizing current. During the second and third week post-natally, these action potentials changed substantially, becoming faster in both their rising and falling phases, shorter in duration, and larger in amplitude. 3. Both mature (greater than P21) and immature (P2-P4) cortical neurones could generate Ca2+-dependent action potentials only if a substantial portion of K+ conductances were blocked. The maximum rate of rise of Ca2+ spikes also increased with age. 4. The apparent input resistance, specific membrane resistance, and membrane time constant all decreased with age from P1 to P30. Immature neurones had I-V relationships that were substantially more linear than those of adult cells, although rectification was often present in both the hyperpolarizing and depolarizing range. Inward rectification in the depolarizing range was Na+ dependent and was substantially larger in mature versus immature neurones. 5. Single, or trains of, action potentials in immature neurones were followed by short duration (10-50 ms) and long duration (1-5 s) after-hyperpolarizations (a.h.p.s) respectively. The duration of the latter appeared to decrease with age. The presence of large a.h.p.s indicates that Ca2+ entry occurs during the action potential of immature, as well as mature, neurones. 6. Responses to intracellular injection of depolarizing current pulses indicated that immature neurones have frequency versus injected current (f-I) relationships which are in general less steep than those for adult neurones and more limited in terms of the range of firing frequencies. 7. Our results are consistent with the hypothesis that there is a considerable increase in the density of voltage-dependent ionic channels underlying the electro-responsiveness of cortical pyramidal neurones during post-natal development.