Thyrotropin—releasing hormone (TRH) was injected into four lactating cows and the effect on serum prolactin and TSH as well as on milk production was determined. Administration of 100, 500 or 1,000 μg TRH resulted in as much as an 11–fold increase in prolactin to 216.4 ng/ml after the 2 higher doses of TRH. Following TRH, TSH increased only 3–fold to 32.4 μU/ml in the group receiving 1,000 μg TRH. Injections of TRH had no significant effect on the yield or composition of milk. (Endocrinology92: 1289, 1973)