Morphine-induced locomotor activity in mice has been investigated but there appear to be only a small number of reports on the effects of narcotic antagonists on this increased activity. Also, it is well known that some narcotic antagonists can act as both morphine antagonists and analgesics in nociceptive assays, but there have been few demonstrations of this stimulant and antagonistic action with locomotor activity studies. In this study, six compounds (cyclazocine levallorphan, diprenorphine (M-5050), nalorphine, naloxone, and naltrexone) have been investigated in regard to their activity as antagonists of morphine-induced locomotor activity and in regard to their ability to stimulate locomotor activity themselves. All six compounds antagonized the effect of morphine, but only cyclazocine and levallorphan produced any significant stimulation of locomotor activity by themselves at the doses tested. This study indicates that changes in mouse locomotor activity can be used successfully to monitor the interaction between morphine and narcotic antagonists and that locomotor activity studies can also be used to study the stimulant (agonist) properties of narcotic antagonists.