Histologically seminal vesicle epithelium (SVE) of the intact adult guinea pig is a discrete and segregated monolayer of highly specialized tall columnar cells. The epithelial layer is so sharply demarcated from its attached stroma (primarily smooth muscle), that blunt dissection alone is sufficient to separate epithelium from muscle. After castration the epithelial cells decrease in both size and number so that by the fifth day, the surviving cells are greatly involuted structurally and comprise only about 12% of the original numerical population normally present in one seminal vesicle. Injected testosterone leads to restructuring of individual cells followed by cell replenishment. The major goal of this effort was to elaborate upon the processes of individual cell growth and cell replenishment during restoration of the tissue to normal cell size and number. The two separate processes were studied using light and electron microscopy, [3H]thymidine incorporation, and Northern blots with labeled histone gene probes. By approximately 48 hr of hormone repletion, parenchymal cell size had returned to normal as the result of a dramatic anabolic process of individual cell growth while cell number remained unchanged. During the subsequent 48-hr period of hormone repletion, the cell population was restored to normal as cell replenishment became the predominant process. Microscopic analysis at intervals throughout the 96-hr period failed to disclose any mitotic events to account for cell replenishment even when Colcemid had been administered. Nor could the increase in cell numbers be correlated with a great increase in [3H]thymidine incorporation or in histone mRNA synthesis. Thus, we could provide no evidence that mitotic division of the parenchymal cells themselves is responsible for cell replenishment. During the 24- to 48-hr interval of hormone repletion, electron microscopic examination disclosed the presence of small epithelial cells lying in a basal position. Some of these cells were seen to insert themselves between the basal regions of parenchymal cells and to expand from the basement membrane into the parenchyma. Possible origins of the cells which replenish the tissue are discussed.