With the aim of studying a possible role of the fibromuscular stroma as growth regulator for the rat ventral prostate the weight, DNA content, and hydroxyproline content were analyzed in the ventral prostate of rats of varying age. Exogenous androgen increased the age-dependent weight and contents only in young and old rats. Orchiectomy reduced prostatic weight, DNA content, and hydroxyproline content although to different extent, and exogenous androgen restored the weight and contents. Exogenous estrogen to intact animals had effects similar to orchiectomy. Estrogen combined with androgen increased DNA content over that found in animals given androgen only, while weight and hydroxyproline content were not changed. It is concluded that the endogenous androgen determines the size of the prostate in young and old animals. Androgen also determines prostatic collagen content. The good correlation between prostatic DNA and hydroxyproline indicates a more or less fixed number of epithelial cells per amount of collagen. A hypothesis suggesting a crucial role for prostatic collagen in the growth-limiting mechanism in the prostate is presented.