SUMMARY Milk-ejection activity (oxytocin) was assayed in the external jugular vein blood of cows, goats and sows during milking or suckling. The hormone was extracted from blood plasma by gel filtration, followed by lyophilization and then assayed in the lactating guinea-pig by the increase in intramammary pressure after intra-arterial injection. Serial blood samples were taken during machine milking of cows which had been accustomed to the milking routine and blood sampling procedure. The stimulus most consistently followed by a transient appearance of oxytocin in the jugular vein blood was the application of the teat cups. In some cases, there was a second release of oxytocin, later in the milking process, unrelated to any apparent stimulus. There was no evidence in these experiments of any conditioning of oxytocin release to visual, auditory or olfactory stimuli associated with the milking routine. Serial blood samples were taken during hand-milking of goats which had been accustomed to the milking routine and blood sampling procedure. Oxytocin was detected in jugular vein blood in only a minority of experimental milkings. Irrespective of whether oxytocin was found in the jugular blood the milk yield at the experimental milking did not differ appreciably from the value to be expected from comparable milkings for the preceding week. Serial blood samples were obtained from sows during suckling, the animals having been accustomed to the presence of the experimenter and the blood sampling procedure. In most of the experimental sucklings oxytocin was found in the blood. The occurrence of the hormone was transient and it was usually released just before the milk-ejection phase. In most of the cases in which no hormone was detected in the blood the piglets obtained no milk as judged by their behaviour. The results suggest that in the cow and sow the milk-ejection reflex is a neurohormonal reflex involving the release of oxytocin. On the other hand, in the goat, provided the animals are carefully hand-milked, normal milk yields can be obtained without the release of oxytocin.