Young mature dogs received finasteride, a selective 5 alpha-reductase inhibitor, orally at 0, 5, 15, and 45 mg/kg/day for 27 or 53 weeks. The effect of finasteride administration on prostatic size and morphology was evaluated macroscopically and microscopically. Changes in glandular and fibromuscular compartments were quantitated by a point counting method on trichrome-stained sections. Finasteride administration induced a decrease of mean prostatic weights and epithelial atrophy in all treated groups. No changes in testicular weights and morphology were observed. The greatest prostatic shrinkage was obtained in the group receiving 45 mg/kg/day for 53 weeks; compared to placebo controls, the percent decreases in absolute volumes occupied by epithelium, lumens, fibrovascular stroma, and smooth muscle were 88, 97, 51 and 72, respectively. These results clearly demonstrate that prostatic shrinkage following finasteride administration results from a decrease in both glandular and fibromuscular compartments.