Fish were adapted to black (B) or white (W) backgrounds for 28 days, or moved between B and W for shorter periods. Compared to W, B induced a darker dorsal surface, the recruitment of new ventral melanophores, and the dispersion of melanin in denervated caudal fin melanophores. These responses were identical in fish adapted to seawater (SW), one-third seawater ( 1 3 SW), or fresh water (FW). The effects of B were abolished (melanophore recruitment, melanin dispersion) or attenuated (dorsal darkening) by hypophysectomy. B did not affect pituitary MSH content or plasma cortisol levels. Examination of the pituitary showed that while the prolactin cells were activated by reduced salinity, only the pars intermedia PAS-positive cell (PIPAS cell) was activated by B, the PI melanotrope (PIPbH cell) and all the pars distalis cells being similar in structure on B and W. Neither the PIPAS nor the PIPbH cell was affected by the different salinities. The neurohypophysis showed no response to background. It is concluded that the melanophore responses to B are mediated by an unknown hormone secreted by the PIPAS cell, and reasons are adduced for believing that prolactin, ACTH, and MSH are not involved in these responses.