(-)-Deprenyl has been increasingly used in recent years as an adjuvant with levodopa and a decarboxylase inhibitor in the treatment of Parkinson's disease. The inhibition of dopa decarboxylase and monoamine oxidase B resulting from this combination suggests that there may be a counter-regulatory increase in the activity of the third main enzyme in the catabolism of levodopa, i.e. catecholamine-O-methyl transferase (COMT). The current study on 36 patients with Parkinson's disease under long-term treatment with levodopa/dopadecarboxylase inhibitor showed, however, that the erythrocyte-COMT was unaffected by additional (-)-deprenyl medication. The patients in this study received levodopa and benserazide either with (-)-deprenyl (n = 21) or without (-)-deprenyl (n = 15). When allowance was made for the different genotypes, COMTLL, COMTLH, and COMTHH, there were no differences in the enzyme activities between the two treatment groups and the untreated controls (n = 26). On the basis of these results, consideration is given to the conditions in which COMT inhibitors are likely to be of value in the treatment of Parkinson's disease.