We studied the effects of estradiol upon the fine structure of the rat ventral prostate, with special emphasis on the interstitial tissue. Intact and castrated rats were injected with estradiol or testosterone in combination, or with peanut oil (control). Estrogen exerted an antiandrogenic effect on the epithelium of intact animals resulting in lower epithelial height and reduction in the number of cell organelles and secretory bodies; however, the prostate gland did not atrophy to the same degree as that in castrated controls. In castrates, estradiol had no discernible effect upon the glandular epithelium. Although testosterone alone was able to restore glandular morphology to normal in these animals, estrogen in combination prevented regeneration of rough endoplasmic reticulum and elicited the formation of large lipid-like inclusions and the accumulation of secretory bodies in the apical zone of many cells. Intact animals treated with these hormones in combination exhibited many of the same effects except that a more normal configuration of rough endoplasmic reticulum was present. In castrates, the prostatic stroma became thickened, with a large increase in fibrous material between and surrounding each acinus, although smooth muscle cells retained their normal cytology. With estradiol treatment, singly or in combination with testosterone, smooth muscle cells increased in size and number and organelles decreased in number, cytoplasm became more electron dense, and nuclei became more heterochromatic. Surface vesicles were profuse in smooth muscle cells in animals treated with estradiol alone; larger phagocytic-like vacuoles were characteristic of the glands of animals treated with estrogen and testosterone in combination. In these latter treatment groups, testosterone prevented the overall hypertrophy and hyperplasia of the interstitial tissue; to a degree, endogenous testosterone levels in intact animals treated with estradiol alone prevented these effects as well.