Estradiol-17β, estrone, estriol or diethylstilbesterol showed the increase in arginase activity of the dorsolateral prostate of intact adult male rats. Neither castration nor adrenalectomy modified the response of arginase activity by administration of estrogen. However, the arginase activity of the dorsolateral prostate of hypophysectomized male rats was significantly reduced regardless of estrogen treatment. On the other hand, almost the similar decrease was observed in the arginase activity of the ventral prostate from castrated, adrenalectomized or hypophysectomized rats, as well as from intact ones.From these results, it was concluded that the increase in arginase activity of the dorsolateral prostate after estrogen treatment might be due to some indirect effects of estrogen, and that pituitary the gland seemed to be involved in this estrogenic effect.