Abstract3 groups of 20 mice were exposed to 4 atmospheres oxygen partial pressure with pure oxygen or nitrogen mixtures, containing 63 or 45% oxygen. The time was measured for the first 10 oxygen fits and for 50% of the animals to become unconscious. With both parameters, if anything, nitrogen seemed to decrease the sensivity of the mice for oxygen poisoning. Provided interspecies differences do not exist, this would imply that decompression‐free diving down to 23 m would be possible. The reasons are discussed for failures in earlier trials to decrease nitrogen partial pressure by adding oxygen to the breathing medium during divin