A polypeptide component,with a molecular weight of 31,200,was separated from whole pituitary of the freshwater species Hypophthalmichthys molitrix by a gel filtration procedure.It was shown to promote release of semen after injection into mature male Puntius gonionotus; its effect was of limited duration. A further component (MW circa 19,700), containing protein and nucleic acid, produced a weak semen releasing effect. A major component of high molecular weight was shown to be inactive.