A clinical trial was carried out in 66 patients to compare the effectiveness of oral flupirtine maleate (100 to 200 mg) and oral pentazocine (50 to 100 mg) in the treatment of pain after hip replacement surgery. The trial analgesics were used as sole analgesia from the second to the fifth post-operative day. Similar numbers of patients were withdrawn from the trial in each group (flupirtine 6, pentazocine 5) because of poor efficacy or the appearance of symptoms, the relationship to treatment of which was uncertain. Indices of the quality, speed and degree of pain relief were similar in both groups on all days of the study, no significant differences being seen. High proportions of patients in each group expressed overall satisfaction with the trial medication, somewhat more so with flupirtine (85% to 95%) than pentazocine treatment (67% to 79%). Reports of dizziness/lightheadedness were significantly more common with pentazocine (23% affected) than with flupirtine (3%). Other side-effects were reported by only small numbers of patients, but the relationship of reported symptoms to treatment was uncertain in most cases. The results suggest that flupirtine is likely to be at least as effective and acceptable as pentazocine for the treatment of pain after orthopaedic surgery and that flupirtine may offer advantages in terms of fewer central nervous system side-effects.