1. In Expt 1 a study was made in goats of responses in terms of milk production, nitrogen utilization and plasma amino acids to abomasas infusions of casein (45 g/d) in goats given 2.5 kg/d of a ration containing crude protein (N x 6.25) at 109 (L1) or 146 (H1) g/kg. 2. In Expt 2 a study was made in goats of responses in terms of milk production, nitrogen utilization, plasma amino acids and growth hormone levels to abomasal infusions of casein (45 g/d) or arginine (25 g/d) in goats given 2.3 kg/d of a ration containing crude protein (N x 6.25) at 104 g/kg (L2). These observations were made also in goats given a ration containing crude protein at 136 g/kg (H2). 3. Milk production in Expt 1 was 2.75, 2.45 and 2.76 kg/d on L1+casein, H1 and H1+casein treatments respectively, the response to casein infusion being significant (P less than 0.05). Milk production in Expt 2 was 1.90, 2.04, 1.96 and 1.96 kg/d on L1, L2+casein, L2+arginine and H2 treatments respectively, and the differences were not significant. 4. Total N intake in Expt 1 was 49, 58 and 64 g/d on L1+casein, H1 and H1+casein treatments respectively. Faecal N was similar on the three treatments (14 g/d), urinary N was 15, 23 and 30 g/d and milk N was 14, 12 and 14 g/d on the respective treatments. Total N intake in Expt 2 was 33, 40, 43 and 44 g/d on L2, L2+casein, L2+arginine and H2 treatments respectively. Faecal N was similar on the four treatments (12 g/d), urinary N was 7, 10, 13 and 14 g/d and milk N was 9, 9, 8 and 8 g/d on the respective treatments. 5. The concentration of indispensable amino acids in plasma was increased by casein infusion in both experiments. It was 1279, 825 and 1133 micrometers/l on L1+casein, H1 and H1+casein treatments respectively in Expt 1, and 1081, 1582, 1055 and 1163 micrometers/l on L2, L2+casein, L2+arginine and H2 treatments respectively in Expt 2. 6. The concentration of arginine in plasma was doubled 1 h after the onset of arginine infusion in Expt 2, Growth hormone levels in plasma were not increased when arginine levels rose following arginine infusion. and did not differ between treatments. 7. The results of the two experiments showed that the stimulatory effect on milk production of intra-abomasal infusion of casein was not reproduced by increasing the dietary intake of protein or by infusing arginine. The results of the second experiment showed that abomasal infusion of arginine did not stimulate production of growth hormone and that growth hormone apparently was not implicated in the effects of casein infusion on milk production.