3H-Melatonin was not metabolized when incubated with rat venous blood in vitro and 72-82% of the added radioactivity was recovered in the plasma fractions. The binding of 3H-melatonin to rat and human plasma proteins was studied by equilibrium dialysis; there were no significant differences between the binding capacities of human and rat plasma, and the proportions bound (77.8 ± 4.3% and 60.7 ± 4.8% at 4 and 37 C, respectively) were independent of melatonin concentration up to 1.5 HIM, indicating a highcapacity binder protein. The in vitro binding of melatonin to plasma proteins was not modified by the presence of other indole derivatives. Equilibrium dialysis of 3H-melatonin with purified plasma protein fractions and electrophoresis of plasma preincubated with melatonin revealed that albumin was the only melatonin-binding protein detectable in plasma. No significant association was observed between melatonin and macromolecules in fresh cerebrospinal fluid (CSF) from goats unless the protein concentration of the CSF was increased by ultrafiltration. Neither the addition of 4% albumin solutions nor of whole plasma modified the melanin aggregation caused by melatonin in melanophores of Rana pipiens larvae, in vivo or in vitro. (Endocrinology91: 1213, 1972)