1. Dermal melanophores of intact Scyllium equilibrate at µ=4.9 and µ=1.4 respectively on black and white background with overhead illumination. They equilibrate at µ=2.9 in complete darkness.
2. Normal responses are of the ‘slow’ type.
3. Expanded melanophores in skin strips immersed in saline contract in 30 min. In pituitary extract they expand in 60 min. So we may interpret the slow responses of intact fish as indices of the slow rise and fall of co-ordinating hormones in circulation.
4. The time relations of normal responses are not consistent with a one-hormone hypothesis.
5. Thyroids removed from animals hypophysectomized a month previously afford no evidence for pituitary control of thyroid activity.