Intracellular recordings in vivo from serotonergic dorsal raphe neurons of the rat brain reveal that these cells undergo a pronounced postspike hyperpolarization followed by a gradual interspike depolarization leading to the succeeding spike. Such repetitive cycles of interspike hyperpolarization and depolarization, which can be termed "pacemaker potentials', can account for the automaticity of these cells. When serotonergic neuronal firing is inhibited by LSD, such pacemaker potentials no longer occur and the cells remain in an hyperpolarized state.