Different areas from brains of 7- day-old female rats were incubated with testosterone- 7-3H (T) or androstenedione- 1-23H (A). Hypothalami from 120-day-old females were also incubated with T. The conversion of these precursors to estradiol (E2) and estrone (E1) was studied using double isotope dilution technique. In addition, the conversion of E2 to E1 and E1 to E2 by brain of the immature females was studied. T was converted predominantly to E2 and A to E1. The yield of E2from T was greatest in incubations of hypothalamus and amygdala, less for the combined septal and preoptic areas and least for the cortex. Conversion of T to E2 by hypothalami of adult rats was less than by that from the immatures. The yield of E1 from A was also greater in hypothalamic than in cortical incubations. There was only minimal conversion less than 3%) of E2 to E1 and of E1, to E2.
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