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1. The initial rates of uptake of carbon dioxide by bovine Hb and HbO₂, in solutions and in intact red cells at 25° C, pH 6·8-7·4, have been measured by the continuous flow rapid calorimeter in the presence and absence of the carbonic anhydrase inhibitor acetazolamide.

2. Over the time range 0-10 msec the only significant reaction, in the presence of acetazolamide, was the carbamino combination of CO₂. In the absence of acetazolamide there was a much larger heat evolution owing to the simultaneous catalysis of CO₂ to bicarbonate due to the enzyme, carbonic anhydrase. Subtraction of the former rate from the latter gives the catalysis of the CO₂ hydration per se. The mode of calculating the actual amount of chemical change from the observed heat evolution is described in detail.

3. The carbonic anhydrase activity was found to vary about threefold between individual blood samples, as in previous work on dilute bovine enzyme solutions, but to be independent of the state of oxygenation of the haemoglobin. Comparison of the enzyme activity in solution and in the red cell seemed to show a discrepancy of the order of 30% in favour of the latter when approximate allowance was made for the difference of pH and Cl^- content in the two cases. This 30% difference might be reduced if exact corrections could be applied.

4. No significant difference was observed between the rates of the carbamino CO₂ reactions in concentrated Hb solutions and red cell suspensions, provided that approximate allowance was made for differences of pH in both cases.

5. The rate of carbamino combination with Hb appeared to be about twice that with HbO₂ at the same pH. Possible explanations for this difference are discussed.