The effect of 3α-androstanediol (3α-diol), 17β-estradiol (E2) and cyproterone acetate (CA) on prostates in castrated beagle dogs were investigated by histologjcal and histochemical examinations. A significant increase of prostatic weight occurred after 6 months’ treatment with 3α-diol alone and in combination with E2. Histologically and histochemically, two different types of prostatic enlargement were observed: first, administration of 3α-diol resulted in diffuse glandular hyperplasia with replacement of functional activity monitored by strongly positive reactions for acid phosphatase, aminopeptidase and zinc. Second, 3α-diol plus E2 produced stratified squamous metaplasia with cystic lumina and loss of the typical morphological structure. These glands showed negative reactions for acid phosphatase, aminopeptidase and zinc. In both types of prostatic hyperplasia CA abolished epithelial or metaplastic proliferation and induced atrophy of glandular epithelium. In estrogenized dogs activation of the fibromuscular stroma was obvious. CA prevented prostatic hyperplasia by atrophying epithelial effects.