Layer V principal neurons of the medial entorhinal cortex receive the main hippocampal output and relay processed information to the neocortex. Despite the fundamental role hypothesized for these neurons in memory replay and consolidation, their dendritic features are largely unknown. High-speed confocal and two-photon Ca(2+) imaging coupled with somatic whole cell patch-clamp recordings were used to investigate spike back-propagation in these neurons. The Ca(2+) transient associated with a single back-propagating action potential was considerably smaller at distal dendritic locations (>200 μm from the soma) compared with proximal ones. Perfusion of Ba(2+) (150 μM) or 4-aminopyridine (2 mM) to block A-type K(+) currents significantly increased the amplitude of the distal, but not proximal, Ca(2+) transients, which is strong evidence for an increased density of these channels at distal dendritic locations. In addition, the Ca(2+) transients decreased with each subsequent spike in a 20-Hz train; this activity-dependent decrease was also more prominent at more distal locations and was attenuated by the perfusion of the protein kinase C activator phorbol-di-acetate. These data are consistent with a phosphorylation-dependent control of back-propagation during trains of action potentials, attributable mainly to an increase in the time constant of recovery from voltage-dependent inactivation of dendritic Na(+) channels. In summary, dendritic Na(+) and A-type K(+) channels control spike back-propagation in layer V entorhinal neurons. Because the activity of these channels is highly modulated, the extent of the dendritic Ca(2+) influx is as well, with important functional implications for dendritic integration and associative synaptic plasticity.