AbstractThe degree of pigment dispersal in the xanthophores and melanophores ofGillichthys mirabilismatched the animal to its background. Hypophysectomy did not affect the melanophores, which appear to be under nervous control, but did cause concentration of the pigment in the xanthophores so that the fish could no longer adapt to a yellow background. Acetyl choline had no effect on the pigment cells but epinephrine concentrated the pigment in the melanophores. Prolactin and high doses of ACTH dispersed the pigment in the xanthophores; the main effect of ACTH was indirect and probably due to stimulation of the interrenal tissue. Cortisol dispersed pigment in the xanthophores and concentrated it in the melanophores. β‐MSH also had an indirect effect in dispersing pigment in the xanthophores. Metyrapone reduced the indirect effect of β‐MSH and ACTH but had no effect on the yellowing of fish to match a yellow background. Thus prolactin, rather than the other hormones, appears to be involved in the normal response to a yellow background. The subcutaneous implantation of a single pituitary provided sufficient hormone to affect the overlying xanthophores. Reserpine resulted in pigment dispersal in the xanthophores of intactGillichthysbut had no effect in hypophysectomized fish. This suggests that the release of prolactin is normally controlled by an inhibitory factor from the