Abstract In earlier studies, we showed that continuous intracerebroventricular infusion of oxytocin, for several days, into the third ventricle of normally hydrated, non-lactating adult female rats significantly reduced glial coverage of magnocellular oxytocinergic neurons in the hypothalamus. It also induced synaptic remodelling whereby many oxytocinergic neurons became synaptically contacted by the same presynaptic terminals (shared synapses). Such changes were closely similar to those observed in the oxytocinergic system when it is physiologically activated, as during parturition and lactation. We now report that central oxytocin does not act alone to modify the ultrastructure of the magnocellular nuclei, but requires the concomitant action of sex steroids. Intracerebroventricular infusion of oxytocin was effective in inducing neuronal-glial and synaptic changes only in supraoptic nuclei of female rats undergoing a prolonged diestrus, or in castrated female rats treated during the infusion period with daily intramuscular injections of progesterone for 4 days followed by 17beta-estradiol for 2 days. Infusion of oxytocin in rats with regular estrous cycles, or in castrated rats treated with progesterone or estrogen alone had no effect on the ultrastructure of the nucleus. Our observations also indicate that the action of oxytocin on the anatomy of its own neurons is very specific: only 4-threonine-oxytocin, a closely related oxytocin analogue, had an effect similar to that of oxytocin; vasopressin, 4-threonine-7-glycine oxytocin and cholecystokinin left the magnocellular nuclei structurally unaltered.