Male pigs were castrated at the ages of 50, 100 and 200 days. Others were not castrated. All were killed at 300 days. The weights of various sexual and endocrine organs at 300 days were determined, and a comparison instituted between pigs castrated at each age. The following results were obtained: (1) The vesicuiae seminales, bulbo-urethrals, and adrenals are smaller when castration is performed at 100 days than when it is performed at 200 days. There is much less difference (possibly none) according to whether castration is performed at 50 or 100 days. In uncastrated pigs the vesicuiae seminales and bulbourethrals are enormously larger even than those of pigs castrated as late as 200 days.(2) It is probable that some change occurs in these organs at about the age of 100 days, rendering them progressively more and more sensitive to the gonad hormone.(3) The thyroid, pineal, and pituitary are not significantly affected by time of castration. Body-weight also is not significantly affected. A general discussion is given on the causes governing the size of the accessory sexual characters in mammals.