1. A new, 2 mu tip, oxygen micro-electrode and a constantly circulated Beckman oxygen gas analyser were used to measure tissue and blood P(O) (2) in anaesthetized, curarized cats under positive pressure breathing. As a parameter for the ability of the circulation to oxygenate tissue, the ;reoxygenation time' (defined as the time required to reach the previous P(O) (2) level after a short period of anoxic anoxia) was determined on blood and cerebral cortex.2. First, it was found that haemorrhage (from 15-25 c.c./kg) alone or haemorrhage combined with sludging of the blood (by the I.V. administration of high molecular weight Dextran, 1 g/kg) markedly diminished P(O) (2) levels in blood and tissue.3. Further, the reoxygenation time was significantly affected by these procedures. Sludging markedly prolonged the reoxygenation time, an effect counteracted by the use of an anti-adhesive drug breaking up the red cell aggregates.4. Bleeding prolonged the reoxygenation time up to four times that found in the same animals previous to the bleeding.