Department of Anatomy, Medical Faculty, Erasmus University, Rotterdam, The Netherlands (Revised manuscript received 28 June 1974) In the rat, unilateral ovariectomy (ULO) performed at dioestrus 2 results in compensatory follicular growth within 24 h (Welschen, 1972). Furthermore, compensatory follicular growth seems to require increased gonadotrophin levels (Greenwald, 1968; Welschen, 1972). Benson, Sorrentino & Evans (1969) and Peppler (1972), using bioassay methods, detected changes in serum gonadotrophin levels 1–4 days after ULO. However, on the basis of the follicular response, more acute changes might be expected. Therefore, it seemed of interest to measure gonadotrophin levels at short intervals after ULO. Wistar rats (180–200 g), which had had at least three consecutive 5-day cycles, were used. ULO or sham-operation was performed under ether anaesthesia at oestrus (12.00 h) or at dioestrus 2 (12.00 h). The rats were bled and killed after a postoperative period of 0-96 h. Blood was obtained under ether