Pituitary glands from immature female and male rats aged between 5 and 30 days were incubated in vitro and the effect of LH releasing hormone (RH) on the release of LH and FSH was studied. Pituitary gonadotrophin contents were also measured. Gonadotrophin release showed changes with age as well as sex differences: after LH-RH stimulation the female pattern of release of LH and FSH (expressed per mg pituitary tissue) showed a peak at day 15; the male pattern of LH release was characterized by a steady increase with age, whereas FSH release stayed more or less constant from day 10 onwards. In both sexes the LH:FSH ratio increased with age, both in pituitary gonadotrophin content and in the mixture of gonadotrophins released. It is discussed, that the prepubertal development of pituitary gonadotrophic function might be determined on the one hand by rather autonomous growth processes (more or less similar in female and male hypophyses) and on the other hand by modulating influences of sex steroid hormones, which are different in female and male animals.