The effect of postoperative pain relief on cardiopulmonary function was examined quantitatively in 36 patients who underwent upper abdominal surgery. Postoperative pain relief was obtained by intravenous meperidine or by mepivacaine epidurally. Expiratory volumes increased during analgesia, despite decreases in respiratory rate and minute volume, especially in the epidural group. The reduction of respiratory rate and minute volume did not result in respiratory acidosis. A reduction of 7 or 8 per cent in oxygen consumption was observed in both groups during analgesia without significant changes in metabolic factors. Differences between decreased &OV0422;s/&OV0422;T in the epidural group and increased &OV0422;s/&OV0422;Tin the meperidine group were statistically significant, After the establishment of analgesia, arterial oxygen tensoin increased in the epidural group and decreased in the meperidine group. The implications of these findings for the problems of postoperative pain relief are discussed.