Histological studies were conducted into the uteri and macroscopic studies into the ovaries of 64 gilts, aged between 180 and 220 days, following administration of 400 I.U. of pregnant mare serum gonadotrophin (PMSG) with 200 I.U. of human chorionic gonadotrophin (hCG) or 500 I.U. of PMSG with 250 I.U. of hCG, in order to test the action of one single injection of PMSG and hCG on the reproductive organs of prepuberal gilts. Ovulation was achieved in something between 75 and 100 per cent of all test animals which were treated in four experimental groups. The mean ovulation rates, following injection of 500 I.U. of PMSG with 250 I.U. of hCG, were between 17.2 and 28.6 (superovulation). The changes induced to the uterus by gonadotrophin injection were similar to those resulting from spontaneous onset of puberty, with the effectiveness of 500 I.U. of PMSG with 250 I.U. of hCG having been stronger than that of 400 I.U. of PMSG with 200 I.U. of hCG. However, histometric examinations have shown that uterus developed with induced cycle was not fully identical with uterus development with spontaneous onset of sexual maturity. Only in about 30 per cent of all animals tested was an induced cycle spontaneously followed by another cycle. Uterus growth was stimulated by gonadotrophin injection even in animals with no further cyclic activity.