To determine the safety, efficacy, and the ventilatory responses to carbon dioxide (CO2) of epidurally administered butorphanol or morphine, 122 healthy women who underwent cesarean section with epidural anesthesia were studied. Patients were randomly assigned to receive one of four epidural regimens for the relief of postoperative pain: 5 mg morphine (n = 32), 4 mg butorphanol (n = 30), 2 mg butorphanol (n = 29), or 1 mg butorphanol (n = 31). Epidural morphine provided satisfactory analgesia with slow onset and long duration of approximately 21 hr. When butorphanol was administered, analgesia of rapid onset was seen with increasing duration and effectiveness observed with increasing dose; approximately 8 hr when using 4 mg. Sixty-two percent of the patients who received morphine had pruritus. Somnolence was the main side effect encountered in patients who received epidural butorphanol. The ventilatory response to CO2 was depressed after morphine and after 2 and 4 mg butorphanol, but the duration of depression was more prolonged after morphine. It is concluded that epidural butorphanol is effective in providing pain relief after cesarean section with minor side effects. However, patients must be observed closely because of possible respiratory depression.