Administration of synthetic TRF, either by intravenous injection or intracarotid infusion to anestrous ewes, caused an immediate and substantial pituitary release of prolactin as indicated by plasma prolactin radioimmunoassay. There was evidence of a dose response relationship between the quantity of prolactin secreted and TRF at levels of 0.2-10 μg/hr by intracarotid infusion or 5-20 μg by intravenous injection. Repeated injections of 20 μg TRF resulted in diminution of the response. The non-specific stimulus of arterial cannulation was also shown to elicit the release of prolactin. Synthetic TRF or arterial cannulation had no effect on plasma LH concentration. (Endocrinology93: 487, 1973)