The skin colour of the cichlid teleost Sarotherodon mossambicus adapted rapidly to changes in background colour. The physiological adaptation was associated with morphological changes in the dermis. Differences in the dermis were found between fish adapted to a black or white background for 14 days. Number and size of the melanophores as well as the amount of pigment in the cytoplasm of the melanophores were significantly increased in fish adapted to a black background. Changes in the dermis parallelled changes in the state of activity of the two endocrine cell types in the pars intermedia of the pituitary. Both the PAS positive cells and the MSH producing cells were more active when the fish were exposed to a black rather than a white background. Fish continuously infused with alpha-MSH, using an osmotic minipump, had more melanophore cytoplasm and pigment per dermis surface unit area than untreated fish. The activity of the MSH cells in MSH-infused fish exposed to a black background was reduced to a level comparable to the MSH cell activity of untreated fish on a white background. alpha-MSH treated fish that were exposed to a white background had many disintegrating MSH cells. These findings point to inactivation of these cells by exogenous alpha-MSH. The activity of the PAS positive cells was not influenced by treatment with alpha-MSH.