Experiments previously reported (Yaginuma, Matsuda, Murasawa, Kobayashi & Kobayashi, 1969) demonstrated that hemicastration on day 0 of life increased the weight of the remaining testis of the rat 72 hr. after the operation. Such an effect might be caused by the secretion of interstitial cell-stimulating hormone and this suggested that the hypothalamo-pituitary testicular axis already functions during the first 3 days of life. Recently, Resko, Feder & Goy (1968) showed that testosterone was present in plasma and testicular tissue during the period from day 0 to day 30 of life. Therefore, the present study was undertaken to ascertain whether the hypothalamo-pituitary testicular axis also functions during the late neonatal period. Rats of the Wistar strain were used. Litters from eight rats were reduced to 8–9 male or 6 female pups soon after birth. On day 5 or day 15 of life, left hemicastration was performed through a lower abdominal incision