Goldfish pituitary gonadotropin-releasing hormone (GnRH) receptors were characterized by using a superagonist analog of teleost GnRH (tGnRH-A; [D-Arg6, Trp7, Leu8, Pro9-NHEt]-GnRH). Equilibrium binding of 125I-tGnRH-A to a goldfish pituitary membrane preparation was achieved after a 30-min incubation at 4 degrees C; binding was significantly reduced after increasing incubation temperature to 22 degrees C. Binding of the radioligand was a function of tissue concentration, with a linear correlation over the range of 0.5-2 pituitary per tube. Incubation of the pituitary membrane preparation with increasing concentrations of 125I-tGnRH-A indicated saturable binding at radioligand concentrations of 470 pM and above. The binding of 125I-tGnRH-A was found to be reversible after addition of the cold analog, and the dissociation curve could be resolved into two linear components; slower rates of dissociation of 125I-tGnRH-A were observed after the addition of excess unlabeled tGnRH than after the addition of tGnRH-A, indicating that the analog is more effective in displacing the label than the native peptide. Addition of the cold analog displaced bound 125I-GnRH-A, and Scatchard analysis suggested the presence of at least two classes of binding sites: a high-affinity/low-capacity site and a low-affinity/high-capacity site. Bound 125I-GnRH-A was displaced by tGnRH from both sites in parallel to that observed with tGnRH-A, indicating that both peptides bind to the same classes of binding sites; however, tGnRH-A had a greater affinity for the receptors than the native tGnRH. These results demonstrated the presence and provided characterization of GnRH receptors in goldfish pituitary.