We have studied the distribution of tyrosine hydroxylase-containing neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the adult human hypothalamus. Large numbers of these neurons were seen in these hypothalamic nuclei; approximately 40% of all the cells within the SON and PVN were immunoreactive for tyrosine hydroxylase (TH-ir). Most of these cells were magnocellular. Their distribution was compared to that of arginine-vasopressin-immunoreactive (AVP-ir) cells. In the SON a greater proportion of magnocellular TH-ir cells was found caudally compared to AVP-ir cells. In the PVN the magnocellular TH-ir cells were larger in mean diameter compared to AVP-ir cells. In double-immunofluorescence experiments some TH-ir cells contained oxytocin immunoreactivity but none contained AVP-ir. In the adult human a large number of PVN and SON magnocellular cells appear to synthesize a catecholamine. A subclass of these neurons also synthesize oxytocin but most cells are distinct from the classically described neurosecretory neurons.