Recent years have seen increased study of dendritic integration, mostly in acute brain slices. However, due to the low background activity in brain slices the integration of synaptic input in slice preparations may not truly reflect conditions in vivo. To investigate dendritic integration, back-propagation of the action potential (AP) and initiation of the dendritic Ca(2+) spike we simultaneously recorded membrane potential at the soma and apical dendrite of layer 5 (L5) pyramidal neurons in quiescent and excited acute brain slices. After excitation of the brain slice the somatic input resistance decreased and the apparent passive space constant shortened. However, the back-propagating AP and dendritic Ca(2+) spike were robust during increased synaptic activity. The dendritic Ca(2+) spike was suppressed by the ionic composition of the bath solution required for slice excitation, suggesting that Ca(2+) spikes may be smaller in vivo than in the acute slice preparation. The results presented here suggest that, under the conditions of slice excitation examined in this study, the increased membrane conductance induced by activation of voltage-gated channels during back-propagation of the AP and dendritic Ca(2+) spike initiation is sufficiently larger than the membrane conductance at subthreshold potentials to allow these two regenerative dendritic events to remain robust over several levels of synaptic activity in the apical dendrite of L5 pyramidal neurons.