The analgesic effect of nefopam hydrochloride was compared with that of pethidine in a double-blind study on postoperative patients with moderate or severe pain. Either 0.3mg./kg. or 0. 5mg./kg. of nefopam, or 1.0mg./kg. or 1.5mg./kg.of pethidine were given by intramuscular injection in a random order to 80 subjects. Pain intensity was assessed on a 4-point scale and pain relief on a 5-point scale, patient were rated before and at 30 minutes, 1, 2, 3, 4, 5 and 6 hours after medication. Pethidine 1.0 mg./kg. and nefopam 0.3 mg./kg. treatments were effective but produced less good analgesia. Time-effect curves in dose of pethidine 1.5 mg,/kg and nefopam 0.5 mg/kg treatments differed somewhat in that nefopam was slower in onset but produced better relief at 6 hours. The incidence of side effect was more after pethidine treatments compared with nefopam treatments. Sweating and anxiety were observed more frequently after nefopam, whereas sedative side effect was more common after pethidine.