The effects of amino acid infusions on plasma levels of prolactin (Prl), growth hormone (GH) and insulin (Ins) in sheep were studied using specific double antibody radioimmunoassay systems.The infusion of arginine (0.5 g/kg), leucine (0.2 g/kg) and phenylalanine (0.3 g/kg) into the carotid artery (via an indwelling cannula) of mature, anestrus ewes, resulted in a significant increase in plasma levels of Prl. Arginine and leucine also stimulated significant increases in plasma GH and insulin levels, while the phenylalanine treatment significantly increased plasma levels of insulin but not GH. The intrajugular injection of arginine (0.5 g/kg) into immature male and female sheep also produced significant increases in Prl, GH and insulin.It was also observed that physically or psychologically stressful factors may produce a significant elevation in plasma levels of Prl, but do not appear to affect GH secretion in sheep. (Endocrinology 9 1 : 205, 1972)