Dendritic spikes signal synaptic integration at remote apical dendritic sites in neocortical pyramidal neurons in vitro. Do dendritic spikes have a salient signaling role under in vivo conditions, where neocortical pyramidal neurons are bombarded with synaptic input? In the present study, levels of synaptic conductance apparent during active network states in vivo were emulated in vitro. Pronounced enhancement of somatic or apical dendritic conductance was spatially compartmentalized and 'visible' over a dendritic length ( approximately 200 microm) on the order of half the voltage length constant, as predicted by passive cable models. The spatial compartmentalization of conductance allowed independent subthreshold synaptic integration at axo-somatic and apical dendritic sites. Furthermore, spikes generated at distal apical dendritic sites efficiently propagated to the axon to initiate action potentials under high synaptic conductance states. The dendritic arborization and voltage-activated channel complement of rat neocortical pyramidal neurons are therefore optimized to allow distributed processing under realistic conductance states.