The testicular hormone, testosterone, is required by the seminal vesicles for the production of fructose, citric acid and other characteristic chemical constituents of the vesicular secretion (see Mann, 1954), and also appears to influence the distribution of water in the gland. Thus, after the injection of testosterone propionate in castrated rats the extracellular water of the seminal vesicle tissue was reduced about threefold, almost to the value of 138 % found in normal, i.e. non-castrated, rats (Rudolph & Starnes, 1954a). The rate of respiration of the tissue from rats treated in this way was also affected; it increased about 30 %, again to the level characteristic of the vesicles from normal rats (Rudolph & Starnes, 1954b).The seminal vesicles of the guinea-pig are different anatomically from those of rats and are shaped in the form of a wide duct which can be opened and the mucosa then conveniently separated as a sheet of tissue. Levey & Szego (1955 a) used such pieces of tissue in a comparison of the metabolism of seminal vesicle epithelia from normal and castrated guinea-pigs, and found that in the latter case the respiration was only a half that of normal tissue or of tissue from castrated animals which had been injected with testosterone. In view of the effects of castration and of testosterone on the water content and respiration of seminal vesicles, a study has been made of the relation between metabolism and the movements of water and ions in the seminal vesicle mucosa of the normal guinea-pig. The concentrations of sodium, potassium and chloride in the tissue, after incubation under various conditions, have been measured and the conclusion has been reached that the tissue can maintain ionic concentration gradients so long as either respiratory or glycolytic energy is available.