The release of antidiuretic hormone in response to the introduction of an osmotic stimulus into the cerebral circulation of the fetal sheep and lamb is described. Infusion of 1.0 M sodium chloride solution (mean 0.22 ml/kg/min, range 0.15–0.42 ml/kg/min) into the carotid artery of the acutely exteriorised fetus led to a rise in fetal plasma arginine vasopressin (AVP) concentration within 5 min. The response was independent of age in the range 113–138 days gestation age. No clear-cut response was obtained in the lamb. In some of the fetal experiments infusion of 0.1 M calcium chloride solution into the carotid artery (mean 0.54 ml/kg/min, range 0.34–0.84 ml/kg/min) led within 2 min to the release of appreciable quantities of both AVP and oxytocin. Turnover measurements of bioassayable AVP indicate a significantly shorter half-life of this hormone in the fetal circulation (3.7 ± 0.7 min, mean ± SEM) when compared with the mother (5.9 ± 0.48 min, mean ± SEM) with an intermediate value in the lamb.