The effects of different backgrounds and illumination levels on somatolactin (SL) concentrations in juvenile red drum (Sciaenops ocellatus) plasma and pituitaries were investigated using a homologous radioimmunoassay. Both plasma and pituitary SL concentrations were significantly higher in red drum exposed for 1 week to black backgrounds than in fish exposed to light backgrounds. Plasma SL levels decreased from 5.39 +/- 1.18 ng/ml in red drum adapted to a dark-blue background to 0.73 +/- 0.05 ng/ml within 1 hr of transfer to light-background tanks. Plasma SL concentrations were significantly elevated in fish 3 hr after transfer from light-background tanks (0.41-0.99 ng/ml) to black-background tanks (5.38 +/- 1.60 ng/ml) or black-background tanks without illumination (3.54 +/- 1.34 ng/ml). Maximum circulating SL levels (9.47 +/- 0.78 ng/ml) were observed 1 day after transfer of red drum to a black-background tank without illumination. Fish exposed to black backgrounds had the darkest body coloration, the greatest dispersion of melanin in their melanophore cells, and the most numerous and active alpha-melanophore-stimulating hormone (alpha-MSH) cells. These results provide evidence that SL may be involved in adaptation of red drum to different backgrounds and illuminations. However, the relative importance of SL and alpha-MSH in the regulation of this process remains to be elucidated.