The paired flank glands of golden hamsters are larger and more pigmented in males than females. In the male, but not in the female, castration results in a sharp decrease in size and pigmentation of the flank gland. The response of the flank gland in the male is linear to the log doses of 0.01-0.25 mg testosterone propionate injected every other day. Testosterone propionate also induces enlargement and increased pigmentation of the female flank gland. Estradiol benzoate is without effect in either sex. The androgen sensitivity of the flank gland makes this organ a useful index of endogenous androgenlevels.