The effects of acute administration of phencyclidine (PCP) on the steady state levels of methionine-enkephalin in discrete brain areas were investigated in mice. The methionine-enkephalin levels in the medulla oblongata-pons and the midbrain were decreased by the administration of PCP. However, PCP induced no change of the methionine-enkephalin levels in other brain areas at the dose range of 5-20 mg/kg. These results suggest that the pharmacological effects of PCP may involve changes in enkephalinergic neuronal activity.