Patient-controlled i.v. administration and intramuscular administration of morphine sulfate were compared in a crossover study to determine their relative effectiveness in relieving postoperative pain. Twenty adult patients scheduled for abdominal surgery were randomly assigned to one of two groups; one group received i.v. morphine sulfate for 24 hours using a patient-controlled analgesia (PCA) device, after which they were given morphine sulfate i.m. for 24 hours. The treatment order was reversed for the other group. Amount of narcotic administered, respiratory rate, and levels of discomfort, activity, and sedation were assessed by the nursing staff every two hours. At the end of each 24-hour treatment phase, patients ranked their level of pain, amount of pain relief, level of sedation, ability to sleep, and ability to perform pulmonary toilet. Patients were also asked whether they preferred PCA or i.m. analgesic therapy for future surgery. Patients reported significantly less discomfort while using PCA than during i.m. morphine administration. No significant differences in amount of narcotic used, respiratory rate, nausea and vomiting, or levels of activity or sedation were noted for the two regimens. Patients' rankings of the two treatment modes did not differ significantly, but a majority of patients indicated a preference for future use of PCA. In these postoperative patients, administration of i.v. morphine sulfate by PCA was as safe as i.m. administration and possibly more effective in relieving pain.