The neurohormone melatonin is a key agent in synchronizing the circadian rhythms. At least three types of binding sites have been described for melatonin: the G-coupled, seven-transmembrane domain receptors mt1 and MT2 and a putative binding site called MT3. The latter has been described in hamster brain membranes, and its binding capacity is optimum at 4 degrees C. We further characterized this binding site on other peripheral hamster tissues, including intestine, liver, kidney, lung, muscle, and heart. We found a high level of binding sites (>30 fmol/mg of protein) in intestine and kidney. Furthermore, we completed the existing pharmacological profile of this site, which can now be described as 2-iodomelatonin > 6-chloromelatonin > methy-isobutyl-amiloride > acridine orange > 5-methylcarbonylamino-N-acetyltryptamine > prazosin > N-acetylserotonin > melatonin. This profile was found in all the hamster organs tested that had a large number of binding sites, namely, brain, intestine, kidney and liver. Furthermore, when comparisons were possible, the MT3 pharmacological characteristics were similar to those described in the literature for hamster brain and testis. This profile was compared to the pharmacology obtained on human cloned mt1 and MT2 receptors and proved to be completely different, as expected. We provide new evidence for an alternate melatonin binding site not only in hamster brain but also in some peripheral organs.