The dendritic arbor of pyramidal neurons is not a monolithic structure.Weshow here that the excitability of terminal apical dendrites differs from that of the apical trunk. In response to ?uorescence-guided focal photolysis of caged glutamate, individual terminal apical dendrites generated cadmium-sensitive all-or-none responses that were subthreshold for somatic action potentials. Calcium transients produced by all-or-none responses were not restricted to the sites of photolysis, but occurred throughout individual distal dendritic compartments, indicating that electrogenesis is mediated primarily by voltagegated calcium channels. Compartmentalized and binary behavior of parallelconnected terminal dendrites can greatly expand the computational power of a single neuron.