Serotonin (5HT) binding sites were studied in goldfish retinal membranes by radioligand experiments. The binding site of [3H]5HT was sensitive to pre-treatment of the membranes at 40 degrees or 60 degrees C. 5HT and 5-methoxy-N,N-dimethyltryptamine were the best inhibitors of [3H]5HT binding to retinal membranes. The 5HT2 agonist, 1-(-naphthyl)piperazine, was also a potent inhibitor, however, (+)-1-2,5-dimethoxy-4-iodophenyl-2-aminopropane was less efficient. The catecholaminergic agents haloperidol and clonidine did not display an important inhibition. Propranolol, also reported as 5HT1B antagonist, was a relatively potent blocker. Monoamine uptake blockers did not show potent inhibition. The GTP analog, GppNHp, inhibited the binding. The iterative analysis of saturation curves revealed two classes of binding sites, a high affinity component (B(max) 2.45 pmol/mg of protein, kd 6.86 nM), and a low affinity component (B(max) 53.46 pmol/mg of protein, Kd 232.07 nM). Analysis of the association and dissociation kinetics suggested a binding site (Kd 2 nM). The semilogarithmic plot of the dissociation kinetics gave curves concave to the upper side. The selectivity of the binding and the inhibition by GppNHp suggest the existence of 5HT1 receptors in goldfish retina. The low affinity interaction probably represents the transporter of 5HT or a subtype of receptor expressed in glial cells.