KW-5338 (domperidone), a new dopamine antagonist, is considered to be an agent to cross the blood-brain barrier with difficulty. The antagonistic activities of KW-5338 against L-DOPA were investigated. KW-5338 showed a strong anti-emetic action against L-DOPA induced emesis in beagle dogs (ED₅₀=0.056 mg/kg (p. o.)) and restored the L-DOPA induced depression of intestinal motility to some extent, while it did not antagonize anti-tremorine activities of L-DOPA and trihexyphenidyl in mice. These results suggest that KW-5338 prevents side effects of L-DOPA such as nausea, vomiting and constipation, without reduction in therapeutic effects of L-DOPA in Parkinson's disease.