1. Arteriovenous differences of plasma free amino acids across the lactating mammary glands of six goats have been measured. 2. In four experiments, measurements of blood flow, amino acid arteriovenous differences, milk yield and milk nitrogen showed that the uptake of nitrogen in the form of amino acids was sufficient to provide all the nitrogen of the milk proteins synthesized in the mammary gland. 3. In the same four experiments the uptake from the plasma and output into the milk of individual amino acids per unit time were compared. The uptakes of essential amino acids and glutamic acid were approximately equal to the corresponding output figures. The uptake of serine was consistently less than the output, and the uptake of other non-essential amino acids was very variable, in some experiments being approximately equal to the output figures and in others being considerably less. 4. As in cows, there was an uptake of ornithine in all experiments, though ornithine is absent from milk. In goats, though not in cows, the uptake of arginine was consistently greatly in excess of the requirement for arginine residues in milk protein. 5. The possible significance of the uptakes of arginine and ornithine for the synthesis of serine and other non-essential amino acids in the mammary gland is discussed. 6. The importance of clamping the external pudic vein, when sampling mammary venous blood from the caudal superficial epigastric vein, is indicated.