Cyclazocine, a benzomorphan derivative, suppressed the flexor reflex of the hind-limb of intact rats induced by intra-arterial injection of bradykinin, a potent pain-producing substance, in a dose-dependent manner, without remarkable influence on motor performance. This suppressive effect was antagonized by naloxone, a specific opiate antagonist. The ED50 values for cyclazocine were 0.054 mg/kg s.c., 5.6 mg/kg p.o., and 1/27, 1/11 of those for pentazocine by the respective routes of administration. In spinal rats, however, the inhibitory effect of cyclazocine and pentazocine on the bradykinin-induced flexor reflex was markedly reduced. Furthermore, cyclazocine as well as pentazocine selectively inhibited the EEG arousal response induced by electrical stimulation of the tooth pulp, which elicits the single sensation of pain. These results indicate that cyclazocine in doses used had a specific analgesic action, and that the main site of action probably is in the supra-spinal structures, such as seen in the case of pentazocine.