SUMMARY: 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 61·0 and 62·3 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 15–17, 11 and 1·8–2·0 g./100 ml. milk respectively, but during the fourth to the sixth week the values for fat and protein increased to 25–30 and 18–20 g./100 ml. respectively, while lactose declined to 0·5 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 0·45 g. body weight/g. of milk consumed, the Dutch litters gained 0·38 g./g. of milk. This difference in weight increment was highly significant (P < 0·001) 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.