The plains mouse has large seminal vesicles, coagulating glands, and dorsal and ventral prostates, whereas in the hopping mouse the only large accessory sex gland is the ventral prostate. Fructose concentration was found to be high in the coagulating glands and dorsal prostate of the plains mouse, but low in all other glands of both species; low concentrations of glucose were found in all glands. Citric acid concentration was high in the dorsal prostate of the plains mouse and ventral prostate of the hopping mouse. Acid and alkaline phosphatase activities were low in all glands of both species. In castrated hopping mice, enlargement of vestigial glands could not be induced with high doses of exogenous testosterone, although there was increased secretory activity of the seminal vesicle epithelial cells.