Radioimmunoassays for alpha-MSH, beta-MSH, ACTH and endorphin were used to measure pituitary concentrations of these peptides in rainbow trout during adaptation to black and white backgrounds. There was no difference in the pituitary content of any of these peptides between long-term black- and white-adapted trout. Plasma levels of alpha-MSH immunoreactivity were significantly higher in black-adapted trout than in white-adapted trout. Time-course studies revealed that although the body colour of trout showed an initial rapid adaptation to background colour, this was not paralleled by a corresponding change in plasma alpha-MSH levels. These only showed significant changes after 7 or more days of background adaptation, when melanophore recruitment or degradation occurred on black or white backgrounds respectively. Intravenous administration of mammalian alpha-MSH, salmon beta-MSH I or antibodies to these peptides did not affect short-term background adaptation. However, long-term administration of mammalian alpha-MSH via osmotic minipump maintained melanophore numbers in grey-adapted trout transferred to a white background, although this observation was based on only two fish. It is concluded that peptides derived from pro-opiomelanocortin do not appear to be involved in controlling physiological colour change but may be involved in regulating morphological colour change of the rainbow trout.