Gold oxygen cathodes, calibrated against a Hersch cell, have been used to monitor oxygen concentrations in the mouse testis. The technique of calibration of the electrodes is described, together with measurements made in mice breathing gas mixtures with different oxygen contents, from 5 to 100%. Mice were found to survive for at least four minutes breathing these mixtures, and the level of tissue hypoxia produced when breathing 10 or 5% oxygen is lower than can be achieved by breathing pure nitrogen. The results show that an oxygen parameter, with dimen- sions of oxygen concentration, may be measured with relatively large electrodes, and is related directly to variations in alveolar pO₂.