Concentrations of cytosolic androgen receptor, DNA and soluble protein, contents of DHT, and in-vivo uptake of 3H-DHT were measured in rat ventral prostates at 5-day intervals during sexual development. Regarding prostate weight two phases of growth were noted being separated by a period of stagnation from Day 40 to 45. Cytosolic androgen receptor, particle-bound DHT, and uptake of 3H-DHT into the 100,000-g sediment showed a clear pattern: a maximum in the prepubertal animal at age Day 20, a minimum at age Day 30 (4 days after the early pubertal rise of LH, testosterone, and DHT) followed by a second maximum on Day 55 (2 days before the beginning of fertility), and a second minimum in the young mature animal on Day 70. An intermediate peak seen at age Day 37 was not significant. Neither the time-dependent profile of the cytosolic androgen receptor nor the contents and in vivo uptake of DHT were correlated to concentrations of circulating gonadotrophins, growth hormone, and sex-steroids measured during puberty in the same strain of animals. Therefore, the regulating mechanism remains unclear.