with oxygen. This can be judged accurately enough if the total bag capacity is not more than about 2 litres. Stage 1?Preparation of the Bag.?The bag of oxygen is rebreathed until an increase is noted in the depth or rate of respiration. About 10 seconds' more rebreathing is encouraged and the first stage is terminated. Stage 2?Fine Adjustment.?After at least two minutes the prepared bag is rebreathed for 20 seconds and its Pco2 is measured. After a further two-minutes rest (about the time required to analyse the previous sample of gas), the 20-second rebreathing period is repeated. This procedure is continued until the bag Pco2 remains unaltered by the rebreathing. This value is taken as the mixed venous Pco2.