This study was conducted to gain further insight into the role of androgen in maintaining a balanced prostate gland growth in dogs. Effects of castration on prostatic cell were assessed by comparing the expression level of high molecular weight cytokeratin (HMW), alpha-actin, and vimentin in intact and castrated dogs. Mature dogs were castrated while they were under general anesthesia and were killed after 1 month. Mature prostate gland structures from intact dogs are characterized by the presence of differentiated columnar secretary epithelial cells and progenitor basal cells that are located within acini and ducts embedded in a thin fibromuscular tissue. Basal cells were distinguished from secretory epithelial cells by HMW cytokeratin immunostaining, which is expressed specifically by basal cells but not by epithelial cells. Castration-induced secretory epithelial cell death, leave the basal cells intact to form a continuous layer lining the atrophied acini. However, the survived basal cells lost their capacity to differentiate to secretory epithelial cells. In addition, androgen ablation induced remarkable reorganization of the cellular components of the fibromuscular compartment. In intact dogs, this compartment of prostate gland is composed mainly of differentiated smooth muscles and scattered mesenchymal muscles as reflected by the high and low actin and vimentin expressions, respectively. Castration for 1 month induced a progressive shift toward mesenchymal cells, which appeared to occupy most of the fibromuscular compartment. Based on these findings, it appears that androgen acts to maintain a steady state of prostate gland by driving the differentiation of prostatic cells and by maintaining its fully differentiated state.