Institute for Research in Reproduction (I.C.M.R.), Parel, Bombay 400012, India (Received 14th January 1975) The increase in the weights of the ventral prostate and other accessory sex organs of the immature rat has been employed as an end point for estimating the biological potency of preparations of LH and HCG (Greep et al., 1941, 1942; Diczfalusy & Loraine, 1955). Several investigators have claimed that prolactin alone or synergistically with androgens has a role in the growth of the male accessory sex organs (Segaloff et al., 1956; Chase et al., 1957; Antliff et al., 1960; Bengmark & Hesselsjo, 1963, 1964; Okamoto et al., 1967; Dorfman, 1972; Negro-Vilar et al., 1973). In these studies, large amounts of prolactin were used, and the possible presence of small amounts of contaminating LH could not be ruled out. It was therefore of interest to re-examine the effects of prolactin alone or with HCG on the