The distribution of radioactivity in control and neonatally androgenized adult rats was determined 48 h after ovariectomy and 1 h after an i.v. injection of either 3H-estradiol or diethylstilbestrol (DES) + 3H-estradiol. Control preoptic-anterior hypothalamic area (POA-AH) and median eminence-basal hypothalamus (ME-BH) concentrated radioactivity 9.6 and 7.9 times more, respectively, than did cortex. Tissue: cortex ratios ranged from 2.3 to 1.0 for other brain areas. Therefore, these two relatively small hypothalamic areas were reproducibly separated by gross dissection from the bulk of the hypothalamus and were shown to have a very high uptake of estradiol. Furthermore, DES inhibited uptake 79% in POA-AH and 85 % in ME-BH, demonstrating that the high uptake in these tissues was mediated by a limited capacity receptor system. DES also inhibited uptake in septum, dorsal hypothalamus and kidney. Radioactivity levels in cortex, hippocampus, brain stem, fat, liver and plasma were not affected by DES pretreatment, verifying that uptake in these tissues was mediated primarily by nonspecific factors. Androgenization did not significantly alter either total uptake or DES-blockable uptake in POA-AH, ME-BH, dorsal hypothalamus, septum or pituitary. The results show that it is possible for persistent-estrous, androgenized rats to have normal total and DES-blockable uptake of estradiol in pituitary and brain areas 1 h after 3H-estradiol injection. Androgenization