The tuberomammillary (TM) nuclei of the hypothalamus appear to be the sole histaminergic cell group in the brain. The extracellular electrophysiological properties of cortically projecting TM neurons were studied in the urethane-anesthetized rat. TM neurons, antidromically activated from either ipsi- or contralateral cerebral cortex, displayed relatively slow conduction velocities, consistent with reports suggesting that TM neurons possess unmyelinated axons. Spontaneous activity was slow and regular, with action potentials of long duration. There was often a noticeable delay between initial segment and somatodendritic portions of spontaneous action potentials, and complete loss of the somatodendritic portion of the second antidromic action potential was commonly seen when double pulse paradigms were employed. These data demonstrate that in addition to anatomical and biochemical similarities, TM neurons share a constellation of physiological properties with other central aminergic neurons.