To get further information on the reason for failure of H. diminuta to grow normally in 9 rats on a diet deficient in vitamin G whereas they grow normally in [female] rats on such a diet, a series of 8 expts., involving a total of 34 groups of 6 rats each, were performed to study the effects on tapeworm growth of castration or spaying, sex hormone administration, pregnancy, and immaturity, alone or in various combinations, when accompanied by complete or vitamin-deficient diets. The results show that castration of [male] rats causes a stunting in the growth of tapeworms even when the diet is complete, but that the growth is normal when testosterone or progesterone is administered to such rats. Spayed [female] rats behaved like normal rats, permitting normal growth when on a complete diet, but stunting when on a vitamin G-deficient diet, with or without administration of [male] or [female] hormones, except for partial improvement in spayed [female][female] given progesterone and in normal [female][female] given theelin, when on a G-deficient diet. Immaturity resulted in partial stunting of growth in [male] rats, but immature [female][female] behaved like adult 9 9 . Only in pregnant rats was fully normal tapeworm growth obtained in [female][female] given a G-deficient diet.