At altitude, bubbles are known to form and grow in blood and tissues causing altitude decompression sickness. Previous reports indicate, that treatment of decompression sickness by means of oxygen breathing at altitude may cause unwanted bubble growth. In this report we visually followed the in vivo changes of micro air bubbles injected into adipose tissue of anesthetized rats at 101.3 kPa (sea level) after which they were decompressed from 101.3 kPa to and held at 25 kPa (10,350 m), during breathing of oxygen or a heliox 34:66 mixture (34% helium and 66% oxygen). Further, bubbles were studied during oxygen breathing preceded by a 3-hour period of preoxygenation in order to eliminate tissue nitrogen before decompression. During oxygen breathing bubbles grew from 11-198 minutes (mean: 121 minutes, ± SD 53.4) after which they remained stable or began to shrink slowly. During heliox breathing bubbles grew from 30-130 minutes (mean: 67 minutes, ± SD 31.0) from which point they stabilized or shrank slowly. No bubbles disappeared during either oxygen or heliox breathing. Preoxygenation followed by continuous oxygen breathing at altitude, caused most bubbles to grow from 19-179 minutes (mean: 51 minutes, ± SD 47.7) after which they would start shrinking or remain stable throughout the observation period. Bubble growth time was significantly longer during oxygen breathing as compared to heliox breathing and preoxygenated animals. Significantly more bubbles disappeared in preoxygenated animals compared to oxygen and heliox breathing. Preoxygenation enhanced bubble disappearance as compared to oxygen and heliox breathing, but did not prevent bubble growth. The results indicate that oxygen breathing at 25 kPa promote air bubble growth in adipose tissue regardless of the tissue nitrogen pressure.