Toward improving the understanding of the etiology of benign prostatic hypertrophy, we have focused on analyses of the hormonal regulation of the male accessory organ fibromuscular stroma, using the guinea pig seminal vesicle smooth muscle. It has been shown that this muscle was a target tissue for both androgen and estrogen, with each steroid causing muscle growth through its respective receptor. Muscle growth responses to both classes of steroids, particularly the accumulations of collagen, were irreversible. In contrast, hormone-induced growths in the epithelium were largely reversible in that the parameters studied declined to the pre-stimulated level following hormone withdrawal. The fact that the muscle was subject to hormone-induced irreversible growths may explain why this tissue represents the predominant component of benign prostatic hypertrophy. These studies also indicate that hormone antagonists such as anti-androgens and anti-estrogens would be effective in preventing, but not reversing, the abnormal accumulations of fibromuscular stroma in this disease.