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The daily milk yields and the concentrations of fat, protein and lactose in the milk were studied throughout lactation in New Zealand White (NZW) and Dutch rabbits. The highest daily milk yields, 270 g. in the NZW and 140 g. in the Dutch rabbits, occurred at the end of the third week of lactation; on a body weight basis the yields were similar being 61\m=.\0and 62\m=.\3 g. milk/kg. body weight for the NZW and Dutch rabbits respectively. Over a 6-week period the total milk yields were 6940 g. for the NZW and 3820 g.for the Dutch rabbits.In both breeds considerable changes occurred in the composition of the milk after the third week of lactation. During the first 3 weeks the concentrations of fat, protein and lactose were approximately 15\p=n-\17, 11 and 1\ m=. \ 8\ p=n-\ 2\ m=. \ 0 g./100 ml. milk respectively, but during the fourth to the sixth week the values for fat and protein increased to 25\p=n-\30 and 18\p=n-\20 g./100 ml. respectively, while lactose declined to 0\m=.\5 g./100 ml. or less.Sheep prolactin (25 i.u. injected s.c. twice daily for 2 days) in late lactation was galactopoietic, augmenting both the daily milk yield and the concentration of lactose in the milk.From days 9 to 31 of lactation the NZW litters gained 0\m=.\45g. body weight/g. of milk consumed, the Dutch litters gained 0\m=.\38 g./g. of milk. This difference in weight increment was highly significant (P < 0\m=.\001) and was apparently due not to a higher calorie content of the milk of the NZW does but to a greater efficiency of milk utilization by the NZW litters.

Twenty-one rabbits were hypophysectomized during the second week of lactation. Hypophysectomy inhibited milk secretion within 3\p=n-\7 days. Restoration of the milk yield to pre-operative levels was obtained when daily injections of either sheep prolactin (50 or 100 i.u.) or human growth hormone (HGH) (2\m=.\5 or 5 mg.) were administered; partial recovery of the milk yield was obtained with 25 i.u. sheep prolactin/day or 1\m=.\25 mg. HGH/day. Neither bovine growth hormone (BGH), adrenocorticotrophin (ACTH) nor cortisol acetate had any significant restorative effects on milk secretion or synergistic effects when combined with sheep prolactin or HGH. The probable development of immune responses to sheep prolactin and HGH was observed. After hypophysectomy, changes in milk composition were found which were similar in trend to the more gradual normal changes observed in late lactation. Sheep prolactin and HGH restored the composition of the milk as well as the yields to normal levels, and both hormones caused rapid increases in lactose yields.The pigeon crop-stimulating potency of the HGH preparation was low (equivalent to 3\m=.\7i.u. sheep prolactin/mg.), yet its lactogenic and galactopoietic potencies were high thus emphasizing the unreliability of assessing the mammotrophic potencies of HGH preparations in birds.

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