Cisapride (CIS) is a prokinetic agent that increases gastrointestinal motility in normal individuals and improves constipation in Parkinson's disease (PD). We studied the effects of CIS on the clinical response and the peripheral pharmacokinetics of orally administered L-dopa given to patients with PD. Twenty patients with idiopathic PD and chronic constipation, whose response to L-dopa was suboptimal or characterized by fluctuations, agreed to participate in an open study that lasted for 2 weeks. Fourteen patients completed the study (mean age 65 ± 9.3 years, mean duration of treatment 5.7 ± 4.2 years, mean L-dopa daily doses 658.9 ± 269.9 mg); six patients were excluded due to lack of compliance or changes in medication during the study. The end points of the study included the mean levels of L-dopa, the height of the peak of L-dopa in plasma, mean plasma levels of 3-OM-dopa, and the speed and quality of gait and visuomanual corrdination before and during treatment with CIS. CIS increased peak plasma levels of L-dopa by 37% and the mean plasma levels of L-dopa by 13% with respect to those obtained with the same dose of L-dopa before the addition of CIS. Therefor, CIS appears to increase early absorption of L-dopa through acceleration of gastric emptying. CIS also increased plasma 3-OM-dopa levels, improved visuomanual coordination, and reduced gait disability. CIS improves gastrointestinal function and response to L-dopa in patients with PD and could be a helpful add-on medication in these patients.