The inactivation of oestrogens in rat uteri was first studied by Lucas, Neufeld, Utterback, Martin & Stotz (1955) and later by Klebanoff (1965) who attributed the reaction to a peroxidase present in eosinophil leucocytes. These cells are not found in the uteri of immature animals but their flow to this organ is influenced markedly by the female sex hormones (Bjersing & Borglin, 1964; Ross & Klebanoff, 1966). More recently, it has been shown (Lyttle & Jellinck, 1972) that an enzyme which is absent from the uteri of immature rats can catalyse the metabolism and binding of [14C]oestradiol to protein in the presence of H2O2 in animals treated with physiological doses of oestrogen or gonadotrophin. Much of the evidence (Jellinck & Lyttle, 1973), coupled with the histochemical studies of Brökelmann & Fawcett (1969) points to the existence of a uterine peroxidase produced in situ distinct from the