The effect of estradiol on serotonin (5HT1) receptors of ovariectomized female rats was studied in discrete brain nuclei by a micro-binding assay and quantitative autoradiography. The two methods show similar distribution of the 5HT1 receptor and estradiol significantly increases the density of receptors in the preoptic area, anterior hypothalamus, lateral septum and arcuate-median eminence. The increase in 5HT1 receptors in these particular nuclei may be related to estrogen control of ovulation and/or other estrogen-mediated central phenomena.