The regulation of oxytocin is incompletely understood and data indicate that in addition to several neurotransmitters, estrogens may be involved. The aim of the present study was to investigate the effects of oophorectomy and hormonal replacement therapy (HRT) on basal levels and 24-h profiles of oxytocin. Basal levels of oxytocin were measured in 95 women who had undergone hysterectomy and who were divided into three groups: group A (n = 30), oophorectomized (BSO), not on HRT; group B (n = 32), BSO, receiving HRT; and group C (n = 33), ovaries preserved and not receiving HRT. The 24-h profiles of oxytocin were measured in nine women before and after hysterectomy. Continuous venous blood sampling was performed 1 week before surgery and 6-7 weeks after surgery for all nine women. Thereafter, three of the four oophorectomized women started replacement therapy with transdermal estradiol 50 micrograms/day. After 10 weeks of treatment, a third sampling was performed. Exogenous estrogen administration was associated with increased oxytocin levels and negative correlation between oxytocin and follicle stimulating hormone/luteinizing hormone levels was found. Removal of the ovaries did not reduce oxytocin levels in any of the investigated groups. When 24-h values were analyzed, no specific rhythmic or pulsatile pattern before or after hysterectomy, with or without simultaneous oophorectomy, was found.