In intracellular recordings from three neocortical pyramidal cells in vitro, intracellular dye injection identified the impalement site as the primary trunk of the apical dendrite. Dendritic recordings displayed two types of regenerative events: relatively fast, low-threshold spikes with amplitudes of 12-69 mV, and slower, higher-threshold spikes up to 80 mV in amplitude. This distinctive dendritic firing pattern was also encountered in six recordings without dye-filled electrodes. Fast spike frequency was extremely sensitive to small changes in membrane potential at the recording site. In one recording, the fast spikes were blocked by 1 microM TTX, while slow events were spared. A computational model of a pyramidal cell was constructed to assist in interpreting the recordings. Simulations suggested that the fast spikes were generated primarily by active Na+ conductance concentrated at a distance from the impalement site, probably in the region of the soma. The low threshold of the fast spikes suggested that Na+ channels also exist in the apical dendrites, where they have a relatively low density. The data strongly imply that there are Ca2+ channels in the apical dendrites.