The relative roles of neural and pituitary elements in controlling the chromatic behaviour of winter flounder, Pseudopleuronectes americanus, are analysed with reference to background-related physiological responses of both epidermal and dermal melanophores and of xanthophores. The effects of spinal section and of hypophysectomy on chromatic responses demonstrate that melanosome aggregation in epidermal and dermal melanophores is under neural control, and that the pituitary has no involvement in melanosome dispersion. However, injection of pituitary extract into hypophysectomized flounder elicited a melanophore concentrating hormone effect, epidermal melanophores being particularly responsive, but its physiological significance has not been established. The differential responses of melanophores, which are associated with patterning in this species, are a function of neural control. In xanthophores the pituitary influence is strong in pigment dispersion, although there is neural mediation in pigment aggregation. These contrasting roles of neural and pituitary elements in controlling melanophore and xanthophore responses provide a basis for both patterning and hue adaptations in this species.