EARLY intracellular studies of cerebral cortical neurons indicated that synaptic input evokes dendritic action potentials that convey information towards the soma(1). Subsequent work in vitro established that neocortical neurons produce dendritic Ca2+ action potentials(2-5). To determine whether natural stimuli elicit Ca2+ spikes, we combined the techniques of whole-cell recording(6,7), pharmacology(8) and quantitative receptive field mapping(9). Our findings show that visual stimulation routinely evoked Ca2+ spikes in distinct functional(10) and anatomical(11) classes of cells in different layers of the cat striate cortex(12). Hence regenerative Ca2+ potentials appear to play a role in both the initial and later stages of cortical sensory processing.