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Xenopus were kept on, and transferred between, black or white back-grounds before incubating the pars intermedia and sometimes other parts of the pituitary with [³H]leucine. The rates of incorporation of label were assessed either by liquid scintillation counting of trichloroacetic acid precipitates or quantitative analysis of autoradiographs. Inhibition of incorporation of label was achieved after incubation with puromycin, showing that the label was associated with newly synthesized peptide material. The rates of incorporation of label into pars distalis and pars nervosa were not significantly different in animals from black or white back-grounds. However, pars intermedia from animals on a black background incorporated label at three to six times the rate of animals from a white background. In animals kept for some days on a white background and then transferred to a black background the rate of incorporation began to rise within a few minutes and reached the new higher level after several hours. In animals transferred from a black to a white background, however, no decline in rate was detected until 2 days after the transfer, but by 3 days the rate had returned to the white background level. The timing of these changes bears no relationship to the timing of changes, reported elsewhere, in the ultrastructure of pars intermedia cells after changes in background colour. They are, on the other hand, similar to the timing of changes in the melanocyte-stimulating hormone content of pars intermedia and also to changes in the melanophore index.