A quantitative radiometric assay utilising incorporation of 14C from labelled precursors as a measure of melanin formation by human brain in vitro is described. The assay was validated by comparison with various criteria of melanin formation. Catecholamines, Dopa and 5Ht were precursors for brain melanin formation. Melanin formation was detected in all brain regions studied and was highest in substantia nigra and striatum. The assay was used to evaluate various hypotheses of brain melanin formation. No evidence for enzymic activity was found and it is concluded that brain melanin formation may be a largely non-enzymic process.