Thirty-six consecutive patients who had undergone major surgery were carefully studied for 6 hours postoperatively in an attempt to evaluate the relative analgesic and respiratory-depressant effect of morphine, ketamine, and placebo administered by continuous intravenous infusion. Side-effects, vital signs and respiratory measurements were recorded hourly by a consultant physician. With regard to the dosage and method of administration, ketamine was shown to be less effective than morphine for the first 3 hours postoperatively, but equally effective subsequently, whereas the patients who received ketamine showed a greater progressive tendency for their respiratory parameters to improve with time. Recommendations regarding the postoperative control of pain are made, and the need for further study is noted.