Departments of Anatomy and Obstetrics and Gynecology, Louisiana State University Medical Center, New Orleans, Louisiana 70112, U.S.A. (Received 31 January 1975) The results of investigations on the effect of unilateral ovariectomy (ULO) in the hypothyroid rat are conflicting because increase in weight of the remaining ovary has been used as the endpoint (Schreiber, Zbuzkova-Kmentova & Zbuzek, 1968; Saiduddin, 1972). A more reliable indication of ovarian function is to determine the number of eggs shed. After ULO, twice as many ova as normal are ovulated from the remaining ovary (Peppier & Greenwald, 1970). In this report the effect of thyroidectomy for either 2 or 7 weeks on the pituitary-ovarian relationship in the rat was studied using the phenomenon of compensatory ovulation after ULO. Virgin, female, Charles River rats were used in two separate experiments. Thyroid glands were surgically removed at 59 days of age under ether anaesthesia. In the first experiment