Binding of 2-[125I]Iodomelatonin to putative receptors for melatonin has been well documented in chicken and hamster tissues. Differences in the potency of N-acetyl-5-hydroxytryptamine in these tissues suggested distinct central and peripheral binding sites. Prazosin was found to be much more potent (Ki = 6–5 nm) in hamster hypothalamus than in chick retina (Ki = 740 nm). N-acetyl-5-HT also was more potent in hamster hypothalamus (Ki = 14 nm) than in chick retina (Ki = 1050 nm). A comparison of hamster hypothalamus to chick forebrain revealed a similar difference in potency for prazosin. In light of these findings, the dissimilarity between the potencies of N-acetyl-5-HT and prazosin in hamster and chick would appear to be due to a species difference as the central and peripheral sites seem alike within either species.