The effects of N-bromoacetyl 5-methoxytryptamine (BraMT) and some related bromoacyl melatonin and tryptamine derivatives on 2-[125I]iodomelatonin (2-[125I]aMT) binding in chicken brain membranes were examined. All analogues displaced specific binding to chicken brain membranes in a concentration-dependent manner. Preincubation of chicken brain membranes with BraMT (10−8M) did not significantly alter either 2-[125I]aMT binding site affinity (Kd) or density (Bmax). SDS polyacrylamide gel electrophoresis of chicken brain membrane proteins preincubated with 2-[125I]BraMT indicated that numerous proteins were labelled. The incorporation of radiolabel into these proteins was not blocked by melatonin, 2-iodomelatonin or N-(l ,4 dinitrophenyl)-5-methoxytryptamine (ML-23), but was reduced by BraMT. In contrast to previous reports on hamster and rat brain membranes, the present results provide no evidence that BraMT or 2-[125I]BraMT affinity labels specific melatonin binding proteins in chicken brain.