The electrochemical oxidation of 5-hydroxytryptamine (5-HT) in acidic solution proceeds through a minor route leading first to 5,7-dihydroxytryptamine (5,7-DHT) then to 4,5,7-trihydroxytryptamine and finally to 5-hydroxytryptamine-4,7-dione. The latter compound is a major electrochemical oxidation product of 5,7-DHT at pH 2 and 7 and a major autoxidation product at pH greater than or equal to 6. Preliminary biological results indicate that 5-hydroxytryptamine-4,7-dione is a more potent central nervous system toxin than 5,7-DHT. These results show for the first time a chemical pathway from 5-HT to 5,7-DHT and suggest possible minor metabolic oxidative pathways for the neurotransmitter 5-HT to at least two powerful neurotoxins.