SUMMARY Cross-circulation experiments were carried out using either two lactating ewes (six experiments) or one sexually mature ram and one lactating ewe (seven experiments). Under local anaesthesia an anastomosis was established by means of two plastic tubes between a jugular vein of one animal (A) and a jugular vein of the second animal (B) in each pair. The other jugular veins were clamped. Vaginal distension performed in ewe A by means of an inflated balloon often caused a sharp rise of pressure in the udder cisterns of ewe B after a minimum latency of 30 sec. Many pressure curves could be obtained successively in the same preparation by repeating the vaginal distension stimulus. Massage of the seminal vesicles and ampullae of the ram often caused a sharp pressure rise in the udder of the ewe after a minimum lag period of 30 sec. This effect could be obtained several times with the same preparation, but considerable individual variation was observed between the different rams used. Massage of the seminal vesicles and ampullae induced emission in many cases but not always. Intravenous injection of a synthetic oxytocin, Syntocinon (500–1000 mu.), in the ram or distension of the vagina in the ewe failed to elicit emission. Pressure responses were induced by injection of suitable doses of Syntocinon into the plastic tube leading to the ewe in which the cistern pressure was recorded, after a latency of approx. 30 sec. It may be concluded that vaginal distension in the ewe causes the discharge of a hormone from the head. A similar phenomenon occurs in the ram after massage of the seminal vesicles and ampullae. These effects are not abolished by atropine. It is postulated that this hormone is oxytocin in both sexes. The amount of hormone released after one stimulus was usually equivalent to 40–100 mu. oxytocin in the ram and 20–50 mu. oxytocin in the ewe.