This randomized, double-blind study compared the analgesic and respiratory effects of lumbar epidural morphine 5 mg, nalbuphine 10 mg, and nalbuphine 20 mg in repeated doses in patients after thoracotomy; the first dose was administered intraoperatively. Pre-and postoperative monitoring included continuous pulse oximetry, respiratory inductance plethysmography, and repeated arterial blood gas analysis. Postoperatively, visual analogue pain scores, somnolence scores, respiratory rate, and arterial blood gases were determined for 16 h. Preoperatively, episodes of apnea were common during sleep but were not associated with low hemoglobin oxygen saturation or increased arterial carbon dioxide tension (PaCO2). During sleep, some otherwise normal patients had increased PaCO2, and 2 of 15 patients had episodes of hemoglobin oxygen saturation of less than 90%. Postoperatively, 1 and 2 h after arrival in the recovery room, patients who received morphine had lower pain scores than did those who received nalbuphine 10 or 20 mg (P less than 0.05). All 6 patients who received morphine had satisfactory analgesia. Two of 4 patients who received nalbuphine 10 mg and all 5 who received nalbuphine 20 mg were withdrawn from the study because of inadequate analgesia (morphine vs. nalbuphine 10 mg, not significant; morphine vs. nalbuphine 20 mg, P less than 0.01). Two patients who received morphine had persistently increased PaCO2 postoperatively. Two patients who received morphine had episodes of apnea and slow respiratory rate, which were most frequent 6 h after arrival in the recovery room. We conclude that lumbar epidural nalbuphine does not provide adequate analgesia after thoracotomy.(ABSTRACT TRUNCATED AT 250 WORDS)