The hypothalamic hormone gonadotropin-releasing hormone (GnRH) has been found in milk of man, cow, and rat. Radioimmunoassays of acidified milk indicate concentrations of GnRH ranging between 0.1 and 3 nanograms per milliliter. Multistep extractions, followed by electrophoresis, reveal gonadotropin-releasing activity in the fraction that comigrates with the GnRH-marker. A second hypothalamic hormone, thyrotropin-releasing hormone, is present in milk at a much lower concentration. "Milk -GnRH" may influence the secretion of the gonadotropic hormones in neonates.