Tonazocine is a nonpeptide, partial delta-opioid agonist with mu antagonist properties and weak k agonist activity. Tonazocine augmented the efficacy of L-DOPA in the reserpine model. Tonazocine evoked a dose-related, ipsilateral rotation, consistent with augmentation of dopaminergic function on the unlesioned side. Tonazocine has a narcotic antagonist activity which is 90 times that of pentazocine and 5 times less than that of naloxone as measured by the antagonism of morphine analgesia in the rat tail-flick test. Total pain relief scores for tonazocine 4 mg were nearly identical with that of morphine sulfate 10 mg while 8 mg of tonazocine were superior to 10 mg of morphine. Drowsiness was the main adverse reaction of tonazocine. Tonazocine has undergone phase II clinical trials for postoperative pain and appeared to possess reasonable efficacy.