A single intravenous injection of tryptophane, 5-hydroxytryptophane, serotonin or melatonin was given to rats on the morning of proestrus and to hypophysectomized, pituitarygrafted female rats. The 5-hydroxytryptophane increased serum prolactin about 9 fold by 30 min after injection and about 6 fold by 1 hr after injection as compared to control values; it also doubled serum prolactin values in hypophysectomized rats with an anterior pituitary graft. Tryptophane approximately doubled serum prolactin over control values by 30 min and 2 hr after injection, but these differences were not significant statistically. Serotonin itself did not significantly alter serum prolactin levels, but melatonin significantly increased serum prolactin over control levels by 1 and 2 hr after injection. These results suggest that brain serotonin, its precursors and melatonin may have a role in stimulating prolactin release, and thereby serve to counter the inhibitory effects of hypothalamic catecholamines on prolactin release. (Endocrinology93: 152, 1973)