Morphine elicited a dose-related increase in the duration of phencyclidine (PCP)-induced motor incoordination. In the open field behavioral observations, morphine enhanced the PCP-induced decrease in the number of ambulation and rearing. Morphine potentiated the PCP-induced decrease in body temperature. The LD50 of PCP was significantly decreased in the presence of morphine. An opiate antagonist, naloxone, antagonized the morphine-induced effects without influencing the pharmacological actions of PCP itself. The levels of hepatic microsomal cytochrome P-450 and cytochrome b5 and the activities of NADPH dehydrogenase and NADPH cytochrome c reductase were unaffected by morphine treatment. The half-lives of PCP in serum and brain were increased by the concurrent administration of morphine. The ratio of the liver weight to body weight and aniline hydroxylase activity in hepatic microsomal fraction were decreased in the morphine-treated group compared with the control group; this is indicative of a possible reduction in the oxidative metabolism of PCP. The results indicate that acute administration of morphine enhances a variety of pharmacological effects of PCP; an inhibition of PCP disposition by morphine may be a mechanism involved in this process.