The purpose of this study was to determine the relationship of epithelial cell types in the ventral prostate glands of castrated mice treated with testosterone during the period of maximum DNA synthesis, mitosis, and restitution of the epithelium. Of 33 adult mice, ten were used as normal controls, six were used as castrated controls at day zero of treatment, and 17 castrates were treated with testosterone for 1 through 8 days. Selected normals, castrates, and treated mice were injected with 3H-thymidine prior to their sacrifice. Tissue samples from each mouse were processed for electron microscopy and autoradiography. The epithelium of the ventral prostate glands after testosterone treatment had four cell types: basal, intermediate, mitotic, and columnar. Intermediate and mitotic cells were most frequent during the second, third, and fourth day of androgen treatments. Basal and columnar cells were observed in normal, castrates, and androgen-treated mice. The restitution of the epithelium included an initial phase of cell growth which was essentially complete by the fifth day of hormone treatment. During the growth phase many basal and columnar cells synthesized DNA and became mitotic. Columnar cells increased their cell height and showed variable amounts of organelles and inclusions. The growth phase was followed by the secretory phase in which columnar cells contained abundant supranuclear organelles and inclusions and appeared as secretory cells much as those observed in normal prostate. The restitution process of the prostate glands of castrates was completed by the eighth day of hormone treatment.