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VARIATION in weight of tissues which respond to hormone stimulation may arise from: (1) variation prior to hormone stimulation, (2) variation in amount of hormone to which the tissue is exposed, and (3) variation in sensitivity of tissues of different individuals. Both initial variation and that due to hormone stimulation are affected by genetical and environmental differences between individuals. This report is concerned with early identification of genetic effects on the weight responses of the testes and the comb. These may, or may not, characterize the responses expected from each chicken throughout its life span. The small amounts of androgen or gonadotrophin which may be present in the blood stream prior to 11 days after hatching would appear to be below the threshold for stimulation of comb and testis size in large bodied breeds. Although Breneman (1941) obtained evidence of gonadotropic and androgenic effects on the testes and combs of …