We examined the membrane potentials and firing properties of motor cortical neurons recorded intracellularly in awake, behaving primates. Three classes of neuron were distinguished by 1) the width of their spikes, 2) the shape of the afterhyperpolarization (AHP), and 3) the distribution of interspike intervals. Type I neurons had wide spikes, exhibited scoop-shaped AHPs, and fired irregularly. Type II neurons had narrower spikes, showed brief postspike afterdepolarizations before the AHP, and sometimes fired high-frequency doublets. Type III neurons had the narrowest spikes, showed a distinct post-AHP depolarization, or "rebound AHP" (rAHP), lasting nearly 30 ms, and tended to fire at 25-35 Hz. The evidence suggests that an intrinsic rAHP may confer on these neurons a tendency to fire at a preferred frequency governed by the duration of the rAHP and may contribute to a "pacemaking" role in generating cortical oscillations.