The study involves an SEM examination of the guinea pig seminal vesicle in both normal and old animals and after orchiectomy. The mucosa showed complicated anastomosing folds. The surface cells were characterized by dense and long microvilli. The density of microvilli decreased in old animals. The lateral surfaces were relatively smooth with few ridges interlocking with the neighbouring cells. The irregular basal surface was covered by basal lamina richly endowed with fine collagenous fibres derived from the loose subepithelial stroma. After orchiectomy the mucosal folds were less prominent. The cells were shorter and with sparse and short microvilli while the subepithelial stroma became more condense. Rejection or degeneration of glandular cells were observed. This shows that the morphology of the luminal surfaces is related to age and stages of function and that castration results in a reduction of microvilli and causes degeneration of cells.