Summary.-Radioimmunoassay methods for the determination of oestradiol-17p and prolactin have been examined for their reliability and applied to the measurement of hormone concentrations in the plasmas from male and female rats. Prolactin was detectable in all samples (>7 ng RP-1 ng/ml) but the concentration of oestradiol-17p was below the sensitivity of the method (>0*10 ng /lOOml) in ovariectomized females. Plasma oestradiol-17p concentration rose gradrfally from metoestrus to proestrus and fell to barely detectable levels in oestrus. Plasma prolactin concentration was very variable even for rats in the same stage of the oestrous cycle but values were minimal in the afternoon of diestrus and a surge in secretion occurred in the afternoon of proestrus. In addition to the stage of the oestrous cycle, the prolactin concentration was influenced by mode of blood collection, degree of haemolysis and choice of serum or plasma. There was no correlation between the concentration of prolactin and that of oestradiol-17P in the same sample of plasma.IN THE RAT, the growth of experimental mammary tumours induced by the administration of the carcinogen 7,12-dimethylbenzanthracene (DMBA) has been reported to be influenced by both oestrogen (Dao and Sinha, 1972), and prolactin (Pearson, 1969). As a preliminary to an investigation of the role of these hormones in tumour growth, we have set up methods for their measurement by radioimmunoassay. The reliability of each method has been examined and the basal levels of each hormone have been measured at various stages during the oestrous cycle, in ovariectomized animals and in males. Although the fluctuations in plasma oestradiol-17/3 (Hori, Ide and Miyake, 1968;Yoshinaga, Hawkins and Stocker, 1969;Brown-Grant, Exley and Naftolin, 1970; Shaikh, 1971;Dupon and Kim, 1973), and plasma prolactin (Kwa and Verhofstad 1967; Niswender et al., 1968;Gay, Midgeley and Niswender, 1970;Amenomori, Chen and Meites, 1970;Neill and Reichert, 1971;Neill, 1972) during the oestrous cycle have been reported previously, the results of simultaneous measurements of both hormones in individual rats have not to our knowledge been published.
MATERIALS AND METHODSAnimals.-Randomly bred SpragueDawley rats were maintained on a lighting; darkness schedule of 12 h L: 12 h D, with the lights switched on at 0700 hours. Vaginal smears were taken daily by lavage and the rats were followed for at least 2 cycles. Only rats with regular 4-day cycles were used and stage in the cycle was defined as previously described (Yoshinaga et al., 1969).Blood was withdrawn into a hepariniZed syringe from the abdominal aorta under ether anaesthesia except when otherwise stated. Blood samples were cooled in ice and the plasma was separated by centrifugation at 4 TC at approximately 1300 g. In one experiment, serum was prepared by the collection of non-heparinized blood, allowing the blood to clot for at least 2 h at 4 TC and centrifugation. Haemolysed plasma was prepared by the addition of water to heparinized blood
