1. Specific high affinity binding of 2-[125I]iodomelatonin was detected in the brain of the pouch young of a marsupial, Bennett's wallaby. 2. Binding was rapid, stable, saturable and reversible. 3. Scatchard analysis indicated a single class of high affinity binding sites with an equilibrium dissociation constant (Kd) of 68 +/- 13 pM, a maximal number of binding sites (Bmax) of 0.7 +/- 0.1 fmol/mg protein and a Hill coefficient (nH) of 1.12 +/- 0.10. 4. Specific binding was inhibited by GTP (1 mM) indicating that the melatonin receptor is coupled to a guanine nucleotide binding protein, and by melatonin and closely related analogues with a potency order identical to that reported previously in the brain of eutherian mammals, birds and a reptile. 5. These studies suggest that the melatonin receptor is well-conserved through evolution.