1. A study was made in goats of the response in terms of milk production, nitrogen utilization, plasma amino acids and amino acid uptake by the mammary gland, to postruminal infusion of casein. Goats in early lactation, housed in metabolism cages, were fed on 2-5 kg basal ration/d (containing 111 g crude protein (N X 6-25)/kg) and were given, by infusion into the abomasum, 0, 15, 30 or 45 g casein/d. 2. Milk production increased from 2-41 kg/d on the basal ration to 2-52, 2-80 and 2-94 kg/d in response to infusions of 15, 30 and 45 g casein/d respectively. Milk composition was unaffected except for milk fat, which was slightly decreased during infusions of the higher levels of casein. 3. The goats were found to be in positive N balance on the basal ration. Milk N output increased with casein infusion; the increase was equvalent to a miximum of 49% of the infused N. 4. The concentration of glucose in arterial blood plasma was decreased at the highest level of casein infusion but that of plasma urea N was unaffected by treatments. 5. Casein infusions increased the concentrations of total indispensable amino acids and the ratio, indispensable: dispensable amino acids in arterial plasma. Arterial concentrations and mammary extractions of most indispensable amino acis were increased, but only a few increases were statistically significant (Pless than 0-05); 6. Comparison of individual indispensable amino acids absorbed from the intestine with output in milk indicated that methionine was probably the first limiting amino acid. 7. The results of the experiment were compared to those of similar experiments with cows that have been reported by other workers. The possible ways in which the infused casein may have caused the responses were discussed. However, no firm conclusions regarding the mechanism involved could be drawn from the results of the present study.