Tritium-labeled 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]pyridinium ion (HPP+) was synthesized enzymatically from [3H]haloperidol using rat liver microsomal preparations, and using prepared [3H]HPP+, the passage of HPP+ into the brain was investigated. Consequently, HPP+ showed a moderate brain uptake index, indicating that it is able to permeate the blood-brain barrier. Furthermore, HPP+ was detected in murine brains after being intravenously injected. These results suggested that HPP+, produced mainly in the liver, is taken up into the brain and induces damage to brain dopaminergic neurons.