The fish Tilapia mossambica were exposed for 30 days to continuously illuminated black or white backgrounds. When compared with the controls the black-adapted fish showed multiplication of the melanophores with a maximal dispersion of the melanosomes. The white background induced blanching of the fish because of depletion of the melanophores and aggregation of the melanosomes. The treatment evoked major changes in the pars intermedia cells. Of these, the amphiphils were stimulated in the black-adapted and regressed in the white-adapted fish indicating that these cells secrete melanophore stimulating hormone. The cyanophils showed depressed activity in the black-adapted fish, but in the white-adapted animals they were stimulated. These alterations suggest the elaboration of a melanophore concentrating hormone by the cyanophils. Regressive effects were also recorded in the gonadotrops and gonads of the experimental T. mossambica.