A method is described for the measurement of oxygen content in whole blood, utilizing an oxygen electrode. The principle relies on the release of combined oxygen into solution when a small volume of blood (100 μl) is added to a relatively large volume (4.9 ml) of 0.4 per cent potassium ferricyanide. From the resulting change in the Po2 of the ferricyanide solution, the oxygen content of the blood can be calculated. Simplification on other similar techniques was effected by the use of micro-syringes and deoxygenated ferricyanide solution. Validation of the method was obtained by comparing results with those obtained using the standard technique of Van Slyke and Neill.