Mean inspired oxygen levels, delivered by the Bennett PR1 and Bird Mark 7 ventilators, were measured at various cycling pressures. Using the Bird ventilator, oxygen levels varied from 51.5 to 96.8%, being highest at low flows and at high cycling pressures. The Bennett PR1 delivers more constant percentages of oxygen, in the range 61.5 to 72.3%. With both makes of machine, each ventilator provides different inspired oxygen levels under comparable circumstances. It is recommended that this type of equipment be driven by compressed air, oxygen being added as required.