Although the formation of seminal fructose is carried out by the male accesory organs, the mechanism of its production is still unknown. The author carried out the experiments to determine the effects of sexual hormones upon fructose in semen using rats by weighing the seminal vesicles and testis. 1. The weights of the seminal vesicles of normal rats were ranging from 9 to 125 mg, whereas the values of fructose were ranging from 114 to 475 mg/100g. From this experiments. the weight of seminal vesicles seems to proportional to the concentration of fructose in semen. 2. Administration of testosterone propionate cause the enlargement of the seminal vesicles and the increase of the concentration of seminal fructose, whereas the prolonged administration of testosterone have and inhibitory action and cause the testicular atrophy. 3. Administration of large quantities of estradiol benzoate have an inhibitory action on seminal secretion and cause the testicular atrophy, the decrease of the concentration of seminal fructose and the weight of testis. 4. Follicle stimulating hormones have stimulating action on testis and cause indirectly the increase of the concentration of seminal fructose. 5. The effects of castration on seminal fructose and the effects of testosterone on the appearance of fructose in castrated rats were significant.