Acetic acid extracts of 339 defatted human hypothalami were subjected to a precipitation procedure and then to separation by several successive steps in order to purify and characterize human releasing hormones.1 Growth hormone-releasing hormone (GH-RH) and FSHreleasing hormone (FSH-RH) were measured by stimulation of release of GH and FSH, respectively, from rat pituitaries in vitro. LH-releasing hormone (LH-RH) and thyrotropin-releasing hormone (TRH) were followed by in vivo assays. The TSH, GH, FSH and LH released were measured by bioassays. LH and FSH were also measured by radioimmunoassays. Gel filtration on Sephadex G-25 separated GH-RH from TRH, LH-RH and FSH-RH. GH-RH was further purified by free-flow electrophoresis and chromatography on carboxymethylcellulose (CMC). TRH, LH-RH and FSH-RH were concentrated by phenol extraction and then chromatographed on CMC, which separated TRH from LH-RH and FSH-RH. In the systems described, as well as others, the behavior of human hypothalamic releasing hormones was similar to or identical with that of hypothalamic releasing hormones from pigs and cattle. LH-RH and FSH-RH of human origin were also tested clinically in humans. It is concluded that hypothalamic releasing hormones are present in man and that their chemical characteristics (molecular weights, isoelectric points, etc.) are very similar to those of domestic animals.