A range of self-administered demand doses of morphine (0.4, 0.7 and 1.0 mg) were prescribed to supplement a fixed-rate infusion (1.5 mg/hour) for pain control after gynaecological surgery. The number of demand doses administered during the first 25 hours after surgery was not significantly influenced by size of demand dose. There was a significant linear trend to increasing amount of demanded morphine with increasing bolus size. These results cast doubt on the usefulness of the concept of a minimum effective analgesic concentration of morphine.