The effects of intrathecal (IT) administration of two doses of morphine (Group 1: 2 mg, n = 9; Group 2: 5 mg, n = 10) were studied in 19 patients after upper abdominal surgery. The ventilatory variables and occlusion pressure (P0.1) were recorded during room air breathing and during CO2rebreathing tests prior to surgery, 24 h after surgery before IT morphine (n = 12), and 3, 5, 7, 11, and 24 h after injection. During room air breathing, minute ventilation (VE) did not change significantly in Group 1 and decreased significantly 3, 5, 7, and 11 h after injection in Group 2. During the rebreathing tests, there was a significant shift to the right of the ventilatory response to CO2in both groups. The peak of the ventilatory depression was delayed, occurring 7 h and 11 h postinjection in Groups 1 and 2, respectively. Two patients in Group 2 developed clinically significant ventilatory depression. The shallow breathing observed after surgery was not changed after analgesia. In group 2, 5, mg IT morphine was responsible for a significant decrease in f60(respiratory frequency for a PETCO2of 60 mmHg). P0.1increased markedly after surgery during both room air breathing and the rebreathing tests. After IT morphine, compared with the postoperative preanalgesic values, P0.160(P0.1at a PetCO2of 60 mmHg) did not change in Group 1 and decreased significantly in Group 2. It is concluded that IT morphine is responsible for a ventilatory depression that is delayed and seems to be dose related and that analgesia does not abolish the shallow breathing observed after upper abdominal surgery.