A study was undertaken to compare continuous subcutaneous infusions of morphine with continuous intravenous infusions in patients whose lungs were mechanically ventilated for 24 hours postoperatively. Serum morphine levels were measured after the end of surgery and at 6, 12, 18 and 24 hours in nine patients receiving continuous subcutaneous morphine and in four patients receiving continuous intravenous morphine given at the same rate. At 6, 12, 18 and 24 hours the means of serum morphine levels in the intravenous group were 20 ng/ml. 17.75 ng/ml, 18.5 ng/ml and 18 ng/ml, respectively, the corresponding figures in the subcutaneous group being 23.2 ng/ml. 20 ng/ml, 20.7 ng/ml and 20 ng/ml. For the intravenous route the mean dose of supplementary analgesia was 14 mg of phenoperidine in the first 24 postoperative hours, whereas for the subcutaneous route the mean dose was 11.66 mg. The differences in the serum morphine levels and in the requirements of phenoperidine were not statistically significant. We conclude that a continuous subcutaneous infusion of morphine is a simple and effective means of achieving postoperative analgesia.