Hohlweg (1934) was the first to demonstrate corpus luteum formation in prepubertal female rats after oestrogen injection. This ' Hohlweg effect' was later confirmed by numerous workers. It is prevented by hypophysectomy (Hohlweg & Chamorro, 1937), or trans-section of the hypophysial stalk (Westman & Jacobsohn, 1938). The present study was made to see whether, in rats of the Wistar strain (Rehbrücke), the Hohlweg effect is based on a sex-specific reaction of the hypothalamo-hypophysial system. Intact prepubertal female rats weighing 40–50 g. were injected s.c. with 12 or 30 μg. oestradiol benzoate (EB) dissolved in 0·2 ml. sesame oil, or with oil alone. Five days later the animals were killed and the uteri and Fallopian tubes removed. The ova were obtained by flushing the uteri with saline and opening the tubes and were counted with the help of a dissecting microscope, the ovaries were fixed in Zenker's solution, sections, 5μ thick, were