The catecholaminergic neurotoxin 6-hydroxydopamine (6-OHDA) has recently been found to be formed endogenously in patients suffering from Parkinson's disease. In this article, we highlight the latest findings on the biochemical mechanism of 6-OHDA toxicity. 6-OHDA has two ways of action: it easily forms free radicals and it is a potent inhibitor of the mitochondrial respiratory chain complexes I and IV. The inhibition of respiratory enzymes by 6-OHDA is reversible and insensitive towards radical scavengers and iron chelators with the exception of desferrioxamine. We conclude that free radicals are not involved in the interaction between 6-OHDA and the respiratory chain and that the two mechanisms are biochemically independent, although they may act synergistically in vivo.