A double-blind, between-patient, two-dose comparison was comparison was performed with pethidine and nefopam in 100 subjects, the majority of whom were recovering from upper abdominal surgery. Either 15 or 30 mg of nefopam or 50 or 100 mg of pethidine were given by i.m. injection in a random order. All assessments were made by the same observer on the first day after operation, at least 4 h after the previous analgesic injection. Nefopam 15 mg was equipotent with pethidine 50 mg, peak analgesia being achieved 1 h after the i.m. injection. Pethidine 100 mg provided significantly better pain relief than nefopam 30 mg, the latter being not more effective than nefopam 15 mg apart from the duration of analgesia which was longer. The incidence of nausea and vomiting was similar after both drugs. Sweating and tachycardia were observed more frequently after nefopam, whereas sedative side-effects were more common after pethidine.