Cholinesterase activity of the prostatic fluid was determi n ed in dogs after administra tion of various hormones. With admini s t ration of male sexual hormones, the activity showed an decrease. With gonadotropic hormones, an decrease was seen in about 10 days of treatment, while a slight decrease was also observed in about 5 days of adrenocortical hormone administration. On the other hand, a marked and immediate increase in cholinesterase activity was seen with administration of female sexual hormones, and similar increase was observed with thyroid hormone administration. Inconsistent change of activity was seen after ACTH therapy and no change was observed after prolactine or anabolic steroids administration.