[125I]-Labelled iodomelatonin binding to brain membrane preparations was examined in adults of 4 species of salmonid fishes; 3 species were of cultured stock (Arctic charr, Atlantic salmon, rainbow trout) and 1 was taken from the wild (coho salmon). The specific binding of 2-[125I]iodomelatonin to the brain membrane preparations collected from all 4 species was specific, stable, saturable, reversible and of high affinity. Competitive inhibition studies showed that only melatonin, 2-iodomelatonin, 6-chloromelatonin and N-acetylserotonin showed significant inhibition of radioligand binding. In the 4 species studied, the equilibrium dissociation constant (Kd) ranged from 9.3 +/- 0.2 pmol/l in coho salmon to 13.2 +/- 0.7 pmol/l in rainbow trout, and those in rainbow trout were significantly higher (p < 0.05) than in the others. The maximum number of binding sites (Bmax) of the Atlantic salmon preparations (14.3 +/- 0.7 fmol/mg protein) was significantly higher than those of the other 3 species (means ranging from 9.9 to 11.6 fmol/mg protein). Hill coefficients of all 4 species were close to unity indicating one class of binding site. The interspecies differences in Kd and Bmax could not be correlated with phylogeny or life history of the various species. For the three cultured species, samples were collected at both mid-dark and mid-light to examine for diurnal variations; none were found.