The binding of 125I-chinook salmon growth hormone (125I-sGH) to rainbow trout brain membranes was studied. Specific binding was detected on telencephalon, thalamus-midbrain, cerebellum-medulla and hypothalamus. In all brain regions, specific binding was dependent on membrane protein concentration, being linear in the range of 250-2500 micrograms of membrane proteins (derived from 30-300 mg wet weight tissue). Scatchard analysis evidenced a single class of high affinity (8.2 +/- 0.3-10 +/- 0.5 x 10(9) M-1) and low capacity (25.8 +/- 6.1-62 +/- 4.5 fmol/g-tissue) GH-binding sites. Specific binding was competitively displaced by recombinant trout GH (rtGH) in a dose dependent-manner. Bovine GH appeared 40-50 fold less potent than cold rtGH for displacing 125I-sGH. Chinook PRL, chinook GtH and bovine FSH did not compete for GH-binding sites in all brain regions examined. Binding studies performed in starved fish indicate that starving conditions decreases the binding of radiolabelled sGH to brain membrane preparations. Our results demonstrate the presence of specific and saturable GH-binding sites on the hypothalamic and suprahypothalamic areas of central nervous system. This finding supports the view that GH plays a direct role on the development and/or function of brain tissue in vertebrate species.