Estradiol-17β,6.7-3H uptake in rat uterus and endometrium was studied at the end of 4-hr intravenous infusions at different phases of the estrous cycle. Different rates of infusion were used. A definite cyclic variation in estradiol-17β,6,7-3H concentration in uterus and endometrium was observed, being lowest at estrus and highest at diestrus, at all infusion rates (from 4.9 to 703 ng/hr). Cyclic changes were parallel in uterus and endometrium. Cyclic variation of estradiol-17β concentration in uterus and endometrium did not parallel the cyclic variation of protein concentration in these tissues. The maximal concentration in uterus, i.e., the “tissue capacity” (NmSu), was found to be: 2.3 ±0.3, 1.8 ±0.2, 2.7 ±0.13 and 2.7 ±0.2 (SD) × l0-8 moles/kg at proestrus, estrus, metestrus and diestrus, respectively. The plasma concentration at half saturation of the uterus (i.e., the “biological exchange constant” (1/Kpu)) was 3.7 ±0.7, 4.5 ±0.6, 3.5 ±0.2 and 3.3 ±0.3 (SD) × 10-10M at these respective phases. Lower uptake at proestrus and estrus may be expected as a result of higher competition of infused estradiol-17β,6,7-3H with endogenous hormone at the uterus level. Lower tissue capacity may partially be explained by a lower cell population at estrus as compared to diestrus. (Endocrinology88: 175, 1971)