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1. Spectrophotometric measurements, using the rapid-mixing constantflow and stopped-flow techniques, have been made of the rate of egress of oxygen from human HbO₂ in solution and in erythrocytes.

2. By 1:20 dilution in a medium containing a high concentration of carbon monoxide k₄, the rate constant of the reaction Hb₄O₈ Hb₄O₆+O₂ has been determined. At pH 7·4 and 20° C, k₄ in solution was 42·0 sec^-1 and within the intact cell 41·4 sec^-1. The respective activation energies were 19·3 and 18 kcal.

3. The rate of dissociation of HbO₂ to Hb, after mixing an erythrocyte suspension with Na₂S₂O₄, was identical, over at least 50% of the reaction, with the change of HbO₂ to HbCO above.

4. An analysis of the data indicates that the cell membrane has little or no resistance to the passage of oxygen and the internal contents are effectively mixed.