Male rats were castrated at 26 (prepubertal), 51 (pubertal) or 76 (postpubertal) days of age. At the same time, they received Silastic capsules filled with testosterone and designed to provide 0 (empty capsule), 10, 20, 40 or 60 mm2 capsule surface area/100 g BW. Sham castrated controls received empty capsules. Four days later on day 30, 55 or 80, the animals were decapitated, secondary sexual organs weighed and serum frozen for radioimmunoassay of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL). When compared with sham operated controls, secondary organs varied in their responsiveness to testosterone as a function of age. The prostates of pubertal animals were the least responsive, while pre- and postpubertal rats showed about the same degree of responsiveness. The seminal vesicles showed a decreasing responsiveness with increasing age. Alterations in serum prolactin concentration did not appear to be responsible for this effect. The sensitivity of the LH and FSH negative feedback systems also altered with age; pubertal and postpubertal animals were less sensitive than prepubertal rats. Altered end organ sensitivity may be an important component of the sexual maturation of the male rat.