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Summary. A specific radioimmunoassay technique was used to measure the concentration of oestradiol at weekly intervals in the blood plasma of seven pullets from 7 weeks of age until they were sexually mature. Large differences in oestradiol concentration were observed between birds. The mean concentration was 94 pg/ml plasma 7 weeks before the first egg was laid. The concentration then began to rise sharply 2 to 3 weeks later to reach a peak of mean value 355 pg/ml 2 to 3 weeks before laying began. The mean concentration when egg- laying was established was 138 pg/ml. The high oestradiol levels found before the onset of laying may be essential for the synthesis of yolk protein precursors and for the conservation of calcium as medullary bone in preparation for egg-shell formation. The peak may also be an important part ofthe changes which occur at puberty in the interrelation- ship between the hypothalamus, the pituitary gland and the ovary.

OESTRADIOL CONCENTRATION IN THE PERIPHERAL PLASMA OF THE DOMESTIC HEN FROM 7 WEEKS OF AGE UNTIL THE TIME OF SEXUAL MATURITY