An experiment was designed to test the effect of castration and testosterone replacement on the development of benign prostatic hyperplasia (BPH) in young and on the restoration of BPH in old beagles. Twenty beagles were divided by age into young (1.5-2.5-yr) and old (6.0-8.5-yr) groups. Each of these groups was further divided randomly into two additional groups of age-matched, intact, untreated control and castrate beagles. The latter were then treated with testosterone-filled Silastic implants designed to clamp serum testosterone at concentrations similar to those observed in adult beagles for 7 months beginning 5 months after castration. Histopathologic characterization of each prostate was completed on biopsy material obtained at the beginning and end of the experiment. Prostate weights were determined each month for 12 months via a noninvasive two-dimensional X-ray procedure. Testosterone treatment for 7 months allowed BPH to develop in young and restored BPH in old beagles. These results suggest that testosterone in the adult beagle acts permissively to allow BPH to develop in the prostate of the aging dog. Some other testicular product may be required for the continued growth of BPH in aged beagles.