Vasopressin (VP) neurons in the suprachiasmatic nucleus (SCN) are thought to be closely linked to neural mechanisms for circadian timekeeping. To gain insight into the cellular-physiological principles that govern spike-driven VP release and to examine whether VP cells can be electrophysiologically and morphologically identified by a unique combination of features, we recorded membrane properties by whole cell patch-clamp methods and stained the cells with biocytin. In current-clamp mode, VP neurons recorded during subjective daytime expressed a clear time-dependent inward rectification but no pronounced low-threshold Ca2+ potential after hyperpolarizing current pulses. Their spontaneous firing rate varied between 0.6 and 13.4 Hz and was generally tonic and irregular. Spike afterhyperpolarizations (AHPs) were steeply rising and monophasic. Spikes were preceded by depolarizing ramps mediated by a slow component of Na+ current. Spike trains evoked by depolarizing current pulses displayed frequency adaptation and were usually followed by an AHP lasting 0.5-2.0 s. Spontaneous postsynaptic potentials were present in a majority of cells. Voltage-clamp recordings revealed a Ba2+-sensitive K+ current that exerts a tonic, hyperpolarizing influence on the membrane potential. This set of membrane properties was not significantly different from other cells in the dorsomedial region and is characteristic for cluster I cells, which were described previously and are widely encountered throughout the SCN. None of the cells could be classified as belonging to cluster II or III, which were indeed found mainly outside the dorsomedial region. Morphologically, single VP neurons were characterized by compact, mono- or bipolar dendritic branching patterns and numerous varicosities throughout the dendrites. They generally possessed few axon collaterals, most of which remained inside the boundaries of the SCN but were occasionally seen to project to SCN target areas. In conclusion, VP neurons in the SCN express several active membrane poperties, including time-dependent inward rectification, frequency adaptation in spike trains, monophasic spike AHPs, and Ba2+-sensitive K+ current. VP release is proposed to be governed by tonic and irregular patterns of spontaneous firing. The electrophysiological and cytological properties of VP neurons are representative for a majority of SCN cells and define them as a subset of previously defined cluster I cells.