The contribution of a hypothetic milk factor in the masculinization process of gonadotropin secretion pattern was investigated using a cross-fostering model. Adult female rats whose nipples had been previously excised were mated. At the time of delivery their pups were given to recipient dams that had given birth one week earlier. Pups remaining with their own (intact) mother served as control group. At the age of 37-39 days (birth = day 0) male rats from the experimental and the control groups were castrated and also control females were ovariectomized. Ten days later gonadectomized animals received ovarian grafts excised from 20-day-old rats. Four and seven days after transplantation the grafts were processed for histology. Corpus luteum formation suggests that male rats nursed by recipient dams did not undergo the masculinization process normally occurring during the first few days of postnatal life. In a separate experiment, male pups nursed by dams being at the early lactation (control) or at the midlactation period were decapitated on postpartum day 1 and serum testosterone levels were measured by RIA. Mean testosterone concentration was almost twice as high in the control group than in pups nursed by recipient dams of the midlactation period. These data suggest that milk of the early lactation period might be necessary for the normal masculinization process of the male rat.