The melatonin-binding sites in membrane preparations of duck kidney were demonstrated by utilizing 125I-labelled iodomelatonin as a radioligand. Binding at these sites was found to be reversible, saturable, specific and of high affinity. Scatchard analysis of the specific binding revealed an equilibrium binding constant (Kd) of 44.6 +/- 4.4 pmol/l (n = 6) and a total number of binding sites (Bmax) of 6.43 +/- 0.60 fmol/mg protein (n = 6) at the mid-point of the light period (mid-light). The Hill coefficient approached 1.0, suggesting a single class of 125I-labelled iodomelatonin-binding site in the duck kidney. Diurnal variation in 125I-labelled iodomelatonin binding showed that the Bmax was 53.4% higher at mid-light than at mid-point of the dark period (P < 0.05), with no significant variation in Kd. The Kd value determined from kinetic analysis was 22.5 pmol/l in birds at mid-light, which was comparable with values determined from equilibrium studies. The order of pharmacological affinity for 125I-labelled iodomelatonin-binding sites in the duck kidney membrane preparations was: 2-iodomelatonin > melatonin > 6-chloromelatonin > 6-hydroxymelatonin > N-acetylserotonin >> 5-methoxytryptamine, 5-methoxytryptophol, 5-hydroxytryptamine, tryptamine, 1-acetylindole-3-carboxaldehyde, 5-hydroxyindole-3-acetic acid, L-tryptophan, 5-methoxyindole-3-acetic acid, 3-acetylindole, acetylcholine, epinephrine, norepinephrine and harmaline. The pharmacological characteristics indicated that 125I-labelled iodomelatonin-binding sites are highly specific for melatonin. Our finding of 125I-labelled iodomelatonin-binding sites in the kidney suggests that melatonin may regulate kidney function.(ABSTRACT TRUNCATED AT 250 WORDS)