1. Evidence is presented that under physiological conditions cysteinyldopa behaves similarly to catecholamines, e.g. adrenaline, in forming reversible complexes with various metallic cations. 2. As a rule, these complexes are stable under anaerobic conditions, but readily autoxidize in the presence of O2 to give the 1,4-benzothiazine acid (II) corresponding to one of the postulated intermediates in the biosynthesis of phaeomelanic pigments. 3. Copper catalyzed oxidation of cysteinyldopa proceeds differently to give the red-purple trichochrome F presumably via decarboxylation and oxidative coupling of the same benzothiazine intermediate. 4. The relevance of these in vitro experiments to the metabolism of cysteinyldopa is briefly discussed.
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