SUMMARY: The uptake of [6,7-3H]oestradiol in vivo and in vitro by cell fractions from regions of rat brain and the anterior pituitary gland has been studied. Cytoplasmic and nuclear receptors were detected in anterior and posterior hypothalamus but not in brain cortex. After labelling in vivo, tissues took up [6,7-3H]oestradiol in the following order of magnitude: anterior pituitary > anterior hypothalamus > posterior hypothalamus > cortex. With the exception of the cortex, all extracts from mature tissues had a higher uptake/mg protein than did extracts from immature animals. In the in-vitro system, oestradiol-17β competed with [6,7-3H]oestradiol-17β in the hypothalamus whereas progesterone, testosterone and oestradiol-17α did not. In the pituitary, oestradiol-17α and 17β competed for binding sites. A single injection of testosterone propionate on the second day of life affected [6,7-3H]oestradiol binding in later life. By 28 days of age, the androgenized animals had a lower nuclear and higher cytoplasmic uptake of [6,7-3H]oestradiol in anterior hypothalamus. This effect was not seen in the posterior hypothalamus or cortex. Binding was decreased in all fractions from the pituitary. In mature animals (60 days old), binding fell in both nuclear and cytoplasmic fractions from anterior hypothalamus and pituitary. The nuclei from posterior hypothalamus also took up less [6,7-3H]oestradiol after androgenization. Androgenization affected specific binding in uteri at both 28 and 60 days of age.