Melanin-concentrating hormone (MCH) is a cyclic heptadecapeptide that concentrates melanin granules in the melanophores and lightens the body color of a fish. To investigate the utility of MCH as a reporter gene, a transgenic medaka strain overexpressing the MCH gene was established and its phenotypic features were examined. The salmon MCH gene driven by cytomegalovirus promoter was injected into 100 fertilized eggs of the HNI-1 medaka strain, which exhibits black body color. One F(0) female transmitted the transgene and a lightened body color phenotype to the F(1) generation. A homozygous transgenic strain was established by crossing F(2) fish homozygous for the transgene. Expression of the transgene was detected in several organs by Northern blotting. The melanin granules of transgenics were highly shrunk. Bioassay using scales confirmed the secretion of MCH into blood, and the MCH concentration was estimated between 0.5 and 5 microM. Development, growth, feeding behavior, and reproduction of transgenics did not differ significantly among transgenic and nontransgenic siblings. The result whereby enhanced MCH expression induced a change in body color, but no remarkable abnormality, suggests the usefulness of MCH as a novel reporter gene with unique features.