Twelve parkinsonian patients with an unsatisfactory therapeutic result on L-Dopa alone due to nausea, vomiting and involuntary movements were treated with L-Dopa and decarboxylase inhibitor. The daily dose reached 800 mg L-Dopa and 200 mg decarboxylase inhibitor. Single doses of each of the components were also given. Electrophysiological examination of hypokinesia, tremor and rigidity, and clinical observation revealed clear evidence of rapid improvement on small doses of L-Dopa combined with decarboxylase inhibitor. Most of the improvement occurred during the 1st week before the maximal dose was reached. A single oral dose of decarboxylase inhibitor resulted in an improvement, suggesting the presence in the organism of a small amount of L-Dopa. This work also shows the absence of liver toxicity of the drug used. Elimination of the extracerebral side effects nausea and vomiting in our opinion is a principle advantage of the compound compared to L-Dopa alone, whereas abnormal involuntary movements, which were found in all patients, remain the limiting adverse side effect.