Abstract The administration of 30–50 μg thyroxine to female rats for 21 days disrupted wheel running activity and cyclic vaginal cornification. Thyroxine produced a decrease in spontaneous activity and the number of activity peaks, and resulted in prolonged vaginal cornification. Following ovariectomy, which reduced activity further, 15 μg/day estradiol benzoate increased overall activity. The data demonstrate that the activity changes produced by thyroxine administration are similar to those observed by others after thyroid removal. In addition, both procedures appear to produce their effects by altering ovarian functioning.