The hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei consist of arginine vasopressin (AVP)- and oxytocin (OT)-synthesizing neurons that send projections to the neurohypophysis, whereas the PVN also projects to other brain areas. A growing body of evidence in animals suggests the presence of sex differences in the vasopressinergic and oxytocinergic systems. The present study was aimed at determining whether the sizes of AVP and OT neurons in the human SON and PVN show sex differences, as earlier studies demonstrated that a change in neuronal size is a sensitive parameter for activity. The minimal and maximal diameters were determined to estimate the volumes of cell somata and cell nuclei in AVP and OT neurons stained with an antibody against human glycoprotein-(22-39), a part of the AVP precursor, and a monoclonal anti-OT antibody in 15 men and 17 women ranging in age from 29-94 yr. The AVP neurons appeared to be larger in young men than in young women (< or =50 yr old). In elderly women (>50 yr old) AVP cell size considerably exceeded that in young women. In elderly men AVP neurons were larger than in young men and elderly women, although these differences were not significant. In addition, AVP cell size correlated positively with age in women but not in men. No significant differences were found in the AVP cell nucleus volumes among all four groups studied. Sex differences in the size of the PVN vasopressin neurons were pronounced at the left side (P = 0.048) and absent at the right side (P = 0.368), indicating the presence of functional lateralization in this nucleus. No difference was found in any morphometric parameter of OT neurons in the PVN among the 4 groups studied. Thus, our data demonstrate sex differences in the size of the AVP neurons, and thus in their function, that are age and probably also side dependent and the absence of such changes in OT neurons in the PVN. These data provide a basis for the reported higher AVP plasma levels in men compared to women.