Developmental patterns of serum levels of androgens and of the luteinizing hormone (LH) were studied comparatively in: i) long-term hyperprolactinemic (LT) immature male rats bearing ectopic pituitary grafts since the age of 21 days and examined at weekly intervals thereafter; and ii) in short-term hyperprolactinemic (ST) males grafted at several postnatal intervals and sacrificed 7 days postoperatively. In both ST and LT rats serum LH was markedly reduced, but only during the early prepubertal period (30 to 37 days), characterized in normal rats by conspicuously high LH levels. During the next pubertal phase of development (51 to 58 days), normally characterized by a steep rise of serum androgens in the presence of relatively low LH, the androgen surge was significantly attenuated in LT, but not in ST animals. This suggests that elevated PRL, if maintained long enough prior to the pubertal age, may significantly attenuate or delay the intensified secretion of androgens characteristic of puberty, presumably by suppressing the intensive prepubertal LH secretion. In LT rats the testicular growth was moderately retarded, but the growth of the dorsal prostate was enhanced, suggesting a PRL-induced increase of responsiveness to androgen.