In a comparison of the effects of domperidone and carbidopa during levodopa treatment, 20 patients with idiopathic Parkinson's disease were treated with fixed dose regimens of either levodopa 500 mg-domperidone 20 mg or levodopa 100 mg-carbidopa 25 mg; each for 8 weeks. Clinical response, incidence of side effects, and plasma levodopa concentration resulting from each treatment were compared. Overall, in the dosages used, Parkinson's disease was less well controlled with levodopa-domperidone than with levodopa-carbidopa. In eight subjects there was a severe deterioration 2 to 7 days after changing from a fixed dose of levodopa-carbidopa to levodopa-domperidone. In nine subjects who completed the trial, the clinical response, occurrence of dyskinesias and of nausea and vomiting, were similar with both treatments, although peak plasma levodopa concentration and levodopa bioavailability were greater on levodopa-domperidone than on levodopa-carbidopa.