A cytosolic oestrogen receptor from baboon endometria was detected and partially characterized. The apparent dissociation constant for oestradiol was 1.5 x 10(-10)--4 x 10(-10) mol/l. Steroids that competed with the [3H]oestradiol binding to the receptor were oestradiol and ethynyloestradiol greater than oestriol greater than oestrone; progesterone, testosterone and corticosterone were not competitors. The [3H]oestradiol-receptor complexes migrated as a 3-3.5S peak during sucrose density-gradient centrifugation when endometrial samples were taken during either the proliferative or the secretory phase. A 7S peak was observed for samples taken at the period of ovulation. A [3H]oestradiol exchange technique was used to detect changes in the receptor concentration during the menstrual cycle. This concentration which was high during the early follicular phase fell sharply before the ovulatory peak of ovarian oestrogens. It remained at a base level during the early secretory phase and then rose during the last days of the cycle to the same concentration as that measured at the beginning of the cycle.

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