Several lines of evidence suggest that endogenous and exogenous toxins may play a major role in the pathogenesis of Parkinson's disease (PD). In the brain aromatic beta-carbolines, like harman or norharman, may be formed by cyclization of indoleamines. Because of the structural similarity to MPTP, beta-carbolines have been proposed as endogenous toxins. For further elucidation of the role of beta-carbolines in neurodegenerative disorders, harman and norharman plasma levels were measured in 36 patients with PD and compared to an age- and sex-matched control group. Plasma levels of norharman in PD were significantly higher compared to the control group. Harman in the plasma of Parkinsonian patients was also elevated compared to the controls, but this difference was not significant. On the one hand these results may suggest a possible role of beta-carbolines in the pathophysiological processes initiating PD. But on the other hand one may speculate that elevated levels of norharman and harman are due to an endogenous upregulation, caused by unknown metabolic processes to reduce oxidative stress by inhibiting e.g. monoaminooxidases in neurons.