Melatonin (N-acetyl-5-methoxytryptamine) is a naturally occurring hormone with a structural resemblance to tryptophan. Its fluorescence is solvent dependent and strongly quenched in the presence of oxygen. Fluorescence quenching in aqueous and organic solutions have been studied in the presence of different quenchers. A 1: 1 inclusion complex formation between melatonin and different cyclodextrins in aqueous solution has been observed. The effects of microheterogeneous media provided by cyclodextrins, micelles and reverse micelles on the fluorescence characteristics of melatonin have been investigated. The water-soluble inorganic quenchers like KBr and KI were unable to quench the fluorescence of melatonin dissolved in a microemulsion consisting of surfactant + cyclohexane + 1-propanol + water, whereas the organic quenchers like CCl4 and CHCl3 were able to quench the fluorescence with a rate constant (on the order of 10(8) dm3 mol-1 s-1) quite similar to that in neat organic solvents.