In cycling rats exposed to a controlled light schedule of 12 h of light daily from 22.00 to 10.00 h the concentration of estradiol in the anterior hypothalamus at 5.00–6.00 h on the day of proestrus was significantly higher than those at 0.00–1.00 h on day 1 of diestrus and estrus, and was about twofold higher than serum estradiol concentration, which reached a peak at 5.00–6.00 h on the same day. Autoradiographically, the uptake of tritiated estradiol by the nuclei of neurons in the nucleus (n.) preopticus, pars suprachiasmatica (POSC), n. preop-ticus periventricularis (POP), and anterior part of n. anterior hypothalami (AAH) was significantly higher at 1.00 h on the day of diestrus 1 than at 6.00 h on the day of proestrus and at 1.00 h on the day of estrus. These results suggest that the POSC, POP and AAH may be target sites in the anterior hypothalamus of estradiol for its feedback action.