The finding that human benign prostatic hyperplasia (BPH) consisted primarily of fibromuscular tissue has led to basic research into the hormonal control of the growth of male accessory sex organ smooth muscle. By using the separated epithelium and muscle layers of the guinea pig seminal vesicle, it was determined that the epithelium exhibited only reversible androgen-induced growths, whereas the muscle proved to be a target tissue for both androgen and estrogen, and exhibited irreversible growth responses. It was of particular interest that the normal androgen-dependent pubertal development of the muscle involved an approximate twofold increase in DNA, followed by the development of a complete and relatively selective androgenic insensitivity in this parameter. An understanding of the factors leading to this apparently normal androgen-dependent loss of the proliferative response in muscle may allow for the development of specific hypotheses for the reawakened stromal growth in BPH. Research on other organ systems focusing on the various factors and mechanisms involved in muscle growth regulation is briefly discussed.