AbstractThe excessive growth of fatty tissue around atrophic organs has been described in humans as “hypertrophy ex vacuo”. Since adipose tissue receives its blood supply from smaller branches of arteries which also supply organs or muscles, it has been suggested that increased amounts of blood could be diverted from the inactive organs to the surrounding fatty tissue and promote its growth. This theory was investigated, using rats and mice of different strains. One testis, the artery of which also supplies the epididymal fat pad, was removed, and the animals sacrificed 3‐4 weeks later. Growth of the affected fat pad in both rats and mice of normal body composition was significantly reduced. Removal of the testis in adult yellow‐obese mice during periods of weight gain produced a moderate but significant weight increase of the affected f