Doses of gonadotropin (HCG) ranging from 0.25 to 20 IU were injected subcutaneously into immature male hamsters daily for 5 days. The increase in weight of the sex accessory glands was used as the end point of the assay. Representative samples of the tissue were fixed in Bouin’s fluid and examined histologically. The weights of the accessory glands increased in response to increasing doses of HCG, up to and including 3 IU/injection/ day. Doses larger than 3 IU caused an inhibition of the weight responses. The increase in weight of the glands was accompanied by cellular maturation, as seen histologically. The anterior prostate is highly sensitive to HCG and the ventral prostate and seminal vesicles are moderately sensitive. The rate of uptake and disappearance of HCG from the bloodstream of young adult female hamsters was determined. Females were injected subcutaneously with 100 IU of HCG. At various times after the injections, samples of blood were withdrawn by cardiac puncture. The serum, containing HCG,...