We have designed a topographic anatomic study using scanning electron microscopy to define surface changes in seminal vesicle epithelium during normal development and after bilateral orchiectomy. Scanning electron microscopy of seminal vesicles from 1-day, 4-week, and 3-month-old intact animals revealed a microvilli-studded luminal epithelial surface with high polyhedral cell profiles extending into the luminal area. Deep furrows outlined all intercellular junctions. Microvilli profiles ranged from short knob-like projections located preferentially along cell margins in 1-day-old animals to 1 mu-long microvilli wih random distribution in mature animals. The most striking finding was that the seminal vesicle epithelium in castrated animals did not revert to the prepubertal state. Rather, cell apices receded from the luminal surface to a level below that of the cell borders and microvilli generally remained somewhat longer in the castrate than in the prepubertal animal. These morphologic findings suggest that the seminal vesicle epithelium in the prepubertal animal and the long-term castrate are quite different.