After 4 hours of unilateral limb ischaemia rabbits develop tourniquet shock, which is characterised by hypovolaemia, a raised potassium level in blood and cardiac failure. Both hypovolaemia and accumulation of potassium indicate deterioration of the cell membrane. Because of the clinical importance of the tourniquet syndrome, direct and local measurement of PO2 and K+ were thought to be of interest. Measurements of PO2 histograms by means of platinum multiwire surface electrodes (Kessler and Lubbers, 1966) were undertaken to study tissue perfusion. In addition extracellular activity of K+ was monitored as a parameter of membrane function with an ion selective surface electrode (Kessler, et al, 1976).