The relationship between hypothalamic catecholamines and anterior pituitary hormone secretion was studied in mature cyclic ewes by the infusion of phenoxybenzamine, and of L—DOPA to alter hypothalamic catecholomines. The treatment effects were assessed by measuring plasma levels of prolactin (Prl) and growth hormone (GH). In addition, an arginine stimulation of Prl and GH was used in combination with phenoxybenzamine and L—DOPA to provide a more sensitive test to possible inhibitory effects of the catecholamines. In positive control experiments with arginine, it was found that the Prl and GH responses to arginine were similar whether arginine was infused on different days, or when two arginine stimuli were administered on the same day in the same group of sheep. The intracarotid infusion of phenoxybenzamine (0.5 mg/kg) resulted in an increase in plasma levels of Prl within 15 min and of GH at 45 min. The treatment with arginine (0.5 g/kg) and phenoxybenzamine (0.5 mg/kg) combined appeared to produce additive effects on Prl and GH. The infusion of L—DOPA (1 mg/kg) alone had no effect on plasma levels of Prl or GH within 60 min. However, when L—DOPA (1 mg/kg) was infused with an arginine stimulus to Prl and GH secretion, the expected increase of both Prl and GH due to arginine was inhibited. Plasma Prl and GH levels did begin to rise 90 and 45 min, respectively, after the combined L—DOPA plus arginine treatment. These results suggest that hypothalamic noradregernic activity may stimulate prolactin inhibiting factor (PIF) secretion. The possibility that hypothalamic norepinephrine may stimulate a growth hormone inhibiting factor (GIF) or that PIF may contain some inherent GIF activity is discussed. (Endocrinology92: 303, 1973)