Two types of cytoplasmic 17 β-estradiol (E2) binding activity were identified and characterized in the uteri of pregnant, cycling and prepubertal, cycle-induced (400 IU pregnant mare's serum gonadotrophin (PMS) + 200 IU human chorionic gonadotropin (hCG)) gilts. Overall, type I affinity and capacity wereKd1.94 ± 0.51 nMand 5.410 ± 1.09 pmol/mg protein, respectively; type II apparent dissociation constant and capacity wereKd21.34 ± 6.83 nMand 62.58 ± 15.96 pmol/mg protein, respectively. Cytoplasmic luteal E2receptors were undetectable in all groups. Uterine E2receptor activity was eluted from diethylaminoethyl columns by a 0.05–0.15 MKCl gradient. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated a molecular weight of 70 400–79 000. Excluding gilts with cystic ovarian follicles (16.67%), prepubertal gilts treated with PMS + hCG versus cycling sows had lower serum progesterone on days 6 and 9–13 of the estrous cycle and lower 13,14-dihydro-15-keto prostaglandin F2αlevels on days 0–9 and 13–17 of the cycle. Implants, containing 200 mg estrone inserted subcutaneously on days 12–19 after PMS + hCG treatment in gilts, had no discernible effects on these parameters. These results indicate that the diminished reproductive capacity of the gilt, in which cycle activity is induced by PMS + hCG, is likely due to decreased luteal progesterone secretion. Preliminary data also suggest that the lack of E2receptors may contribute to the low reproductive performance in gilts with cystic ovarian follicles.