Testis weight and seminal vesicle weight (relative to total body weight) in rdw rats and N rats were compared with advancing age. The results obtained were as follows 1) The increase in testis weight became remarkable at 15-16 weeks of age. The weight remained constant up to 23-30 weeks of age and decreased thereafter with age. 2) Seminal vesicle weight increased rapidly in rdw rat at 15-16 weeks of age, reflecting elevated androgen secretion and sexual maturation. 3) From the viewpoint of histology, one of causes of the increase in testis weight may be edema. The increase in testis weight coincided with the increase in androgen secretion at 15-16 weeks of age as suggested by the relative increase in seminal vesicle weight.