In mammals, the magnocellular neurons of the supraoptic nucleus (SON) have been classified into vasopressin- (VP) and oxytocin- (OT) producing subtypes. The degree to which these neurons have distinguishable characteristics is considered in the present review. Most of the cytoarchitectonic diversity observed in some Golgi studies has yet to be attributed to differences between OT and VP neurons. The predominant SON cell type is a large bipolar neuron with relatively short and simply branching dendrites. Based on intracellular filling, large multipolar neurons probably represent a small subset of neurosecretory cells. Parvicellular multipolar and bipolar neurons may represent interneurons or subsets of neurosecretory cells. Suggestive evidence that axonal origin and spine density may differ between OT and VP neurons remains to be confirmed in rat. Different fiber systems are thought to preferentially innervate VP or OT subgroups, but only rarely have inputs to OT and VP neurons been compared at the ultrastructural level. Potentially selective inputs to OT somata may derive from the raphe system and the nucleus of the solitary tract, whereas the apparent preferential innervation of VP neurons (e.g. from the A1 region of the ventrolateral medulla) is less certain because of the overlapping dendritic fields of OT and VP neurons. Electrophysiologically, OT and VP neurons are best distinguished in vivo by their reaction to gastric, cardiovascular and suckling stimuli. The firing patterns of activated OT and VP neurons often differ, but can transiently appear indistinguishable in vivo and especially in vitro. Classification in vitro without immunochemical labelling may be aided by the presence of phasic bursting (mostly in VP neurons) and by the differential response of these neurons to certain neurochemicals or to stimulation of certain inputs. The membrane properties of OT and VP neurons are generally similar in vitro, but the range of tests has not been extensive. The depolarizing afterpotential is more often exhibited by, but is not exclusive to, VP neurons.