Injected air bubbles in spinal white matter in the rat were studied at 1 bar after decompression from an exposure to air at 3.1 bar (absolute) for 4 h. During air breathing all injected bubbles grew for the first 2 h of the observation period. Thereafter three of nine bubbles began to shrink and one of them disappeared. During breathing of heliox (80:20) bubbles consistently shrank and disappeared from view. If the breathing gas was changed from heliox to N2O-O2 (80:20), while bubbles still had an appreciable size, they started growing again. If the change to N2O-O2 was done after a bubble disappeared from view, it did not reappear. During breathing of 100% oxygen, all bubbles initially grew. Subsequently they all shrank and disappeared at about the same time after gas shift, as during heliox breathing. The effect of heliox treatment on CNS decompression sickness after air dives is discussed.