The response of the immature rat uterus to the antiestrogen, nafoxidine (Upjohn U-11, 100A), is maximum at a dose of 5 micrograms. This dose of nafoxidine sustains the uterine response for at least 72 h. After treatment with 5 micrograms nafoxidine for 24 h, uterine cytosol contains approximately one half the total number of estrogen receptors originally present in uteri of untreated animals, and uterine nuclei also contain approximately one half the receptors originally present in unstimulated tissue. The total amount of uterine estrogen receptor, however, is not altered 24 h after treatment with nafoxidine relative to saline-treated controls, while estradiol treatment doubles the total amount of receptor relative to controls. Pretreatment with 5 micrograms nafoxidine for 24 h does not block the uterine response occurring 4 h after the subsequent administration of estradiol, but does block the uterine response occurring 24 h after the subsequent administration of estradiol. These results suggest that uterine nuclei contain different acceptor sites for regulating short term and long term uterine responses to estrogen.