The aim of this study was to assess the quality of postoperative analgesia obtained with morphine-ketamine association administered in self-pain controlled analgesia, as well as the amount of morphine that has been spared. Patients who had to undergo thoracic surgery were selected. They were divided into two groups: G1 was made up of patients receiving 0.5 mg/ml of morphine associated with a placebo, with boluses of 2 ml and refractory periods of 5 minutes ; and G2 made up of patients receiving 0.5 mg/ml of ketamine associated with 0.5 mg/ml of morphine with same boluses and refractory periods. The assessment of pain at rest and on stimulation was carried out with the visual analogue scale. The response to pain and the amount of morphine spared were evaluated. Fifty patients with an average age of 34 years were selected. The assessment showed that the response to pain at rest was the same in the two groups as from the twelfth hour. On stimulation, the analgesic response was better in G2 as well as the amount of morphine spared. This study shows that the administration of ketamine in association with morphine in the post operative period procures a favourable efficiency-tolerance relationship and provides a good means of sparing morphine.