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Summary Anaesthetized dogs subjected to severe haemorrhagic hypotension followed by spontaneous uptake of 30 per cent of the shed blood were studied. One group was then transfused with blood, another with low molecular weight dextran, and a third with Locke's solution. During the hypotensive phase, the arterial p H, standard bicarbonate, buffer base and Pco 2 fell, while the lactate, pyruvate, and potassium values rose. Within half a minute of retransfusion, the p H decreased further but the Pco 2 rose to normal or higher values. The greatest fall in p H occurred in the dextran group, but the p H of dextran is lower than that of the reinfused blood or Locke's solution. The fall in p H and rise in Pco 2 after retransfusion occur with unchanged lactate levels and so cannot be attributed to ‘washout’ of lactate from the tissues.

Dogs under chloralose anasthesia were bled at a rate of 50 ml/min to a total of 25 ml/kg body weight and 2 minutes later a quick reinfusion of adequate volumes of blood, dextran, or Locke's solution was done. Within 2 minutes after reinfusion, the pH of arterial blood fell by 0.074--0.127; concurrently, PaCO2 rose by 9.2-12.9 mm Hg. A close correlation was demonstrated between these changes. After retransfusion, PaO2 and the arterial lactic acid level did not change significantly. Thus retransfusion acidosis in the dog appears after a brief hypotensive period, too, but cannot be attributed to a "washout" of lactate from the tissues.

The blood pressure of dogs anaesthetized with 0.1 g/kg of chloralose was reduced to 40 mm Hg by arterial bleeding. Thirty percent of the drawn blood refluxed spontaneously, the rest was retransfused either suddenly arterially or slowly in 15 minutes by the venous route. In other groups a total quantity of 25 ml/kg was drawn from an artery at a rate of 50 ml/kg and after about one minute the entire quantity of blood was rapidly retransfused through the artery. The control group breathed spontaneously, the other groups were given artificial respiration, or the sinus nerves and the vagus were transected separately or simultaneously, or transection was combined with artificial ventilation.