The activity of oestrogen-induced peroxidase in sections along the uterus of normal and mammary tumour-bearing adult rats was measured. Oestradiol increased the activity of this enzyme in the cervix as well as in other parts of the uterus in ovariectomized or immature rats. Peroxidase activity per mg protein was twice as high in the cervix as in the rest of the uterus where it was evenly distributed along both horns. The concentrations of oestrogen receptors in the cytosol and nucleus in each uterine horn and in the cervix was also determined and found to be lower in the cervix than in other sections of the uterus.The rapid increase in uterine peroxidase activity after treatment of ovariectomized or immature rats with oestrogen (Lyttle & Jellinck 1972; Jellinck & Lyttle 1973) has led to the proposal Lyttle & DeSombre 1977) that this enzyme may be a useful specific marker for tissues whose growth is regulated by oestrogen. In recent studies, Tsibris et al. (1978a) determined the distri¬ bution of oestrogen and progesterone receptors in the cytoplasm of five sections along the length of the human uterine endometrium obtained from non-cancerous pre-menopausal hysterectomy spe¬ cimen. They also measured the peroxidase activity of these sections (Tsibris et al. 1978b). The concen¬ tration of both types of steroid receptors was found to be highest in the fundus and lowest in the cervix while the enzyme was located either exclusively or maximally in the cervix. It was therefore con¬ sidered of interest to measure peroxidase activity along the length of the rat uterus and also to compare the concentration of nuclear oestrogen receptors in the cervix with that of the rest of the uterus. Materials and MethodsDetermination of uterine peroxidase activity Mature female Sprague Dawley rats (Canadian Breeding Laboratories, St. Constant, Quebec) weighing 290-320 g were injected sc with oestradiol-17ß (10 ug m 0.5 ml saline containing 10% ethanol) 10 h before sacrifice. Six of the animals used bore mammary tumours induced by the gastric instillation of a single dose (15 mg) of dimethylbenz(a)anthracene (DMBA). Uteri (499 ± 37 mg) were removed, cut into 1-1.5 cm sections with scissors as illustrated in Fig. 1 and each section weighed and homo¬ genized in 3 ml of Tris-HCl (0.01 m) pH 7.2 in a Potter-Elvehjem homogenizer with a Teflon pestle. The homogenate was centrifuged at 40 000 g for 30 min and peroxidase solubilized by re-homogenizing the sediment in Tris-HCl buffer containing 0.5 M CaCl2 (Lyttle & DeSombre 1977). It was centrifuged again for 30 min and the supernatant (0.05-0.2 ml) used for determining peroxidase activity by measuring the rate of oxidation of guaiacol at 470 nm (Himmelhoch et al. 1967). The concentration of protein in the uterine extracts was determined by the Bradford dye-binding method using bovine y-globulin (Cohn fraction II) as standard (Brad¬ ford 1976). In some experiments, mature rats ovariecto¬ mized under methoxyflurane (Metofane) anaesthesia 14 -21 days before sacrifice, and also immature (22-26 da...