After estradiol (E2) administration, early increases (within 4 h) in uterine wet weight and the synthesis of 2-deoxyglucose-6-phosphate from 2-deoxyglucose are similar in ovariectomized rats and in ovariectomized rats made hypothyroid by feeding a low iodine diet containing prophylthiouracil. Most late uterine responses occurring 24 h after E2 treatment, however, are greatly diminished in the hypothyroid animals. The diminished responses include increases in uterine wet weight, dry weight, protein content, RNA content, and the incorporation of thymidine into uterine DNA. One response, the synthesis of 2-deoxyglucose-6-phosphate from 2-deoxyglucose is not diminished in hypothyroid rats 24 h after E2 treatment. The diminished uterine response is not due to a shift in the dose-response curve for E2, but results from a decrease in the magnitude of the maximum uterine response. Treatment of hypothyroid rats with exogenous T3 restores the diminished uterine response in a dose-dependent and time-dependent manner. A dose of 0.5 microgram T3/100 g BW for 5 days restores the response completely, while 48-72 h of treatment with higher doses of T3 are required to restore the response. The effect of T3 is not mediated by the pituitary, since exogenous T3 restores diminished uterine responses in ovariectomized, hypophysectomized animals. These results suggest that thyroid hormones may have a direct effect on the uterus which regulates the responsiveness of the organ to E2.