Adult male rats were individually accustomed to experimental conditions in 16X16X30 cm polymethacrylate chambers. Each of 12 rats was twice subjected during 5 hrs or 5 1/2 hrs to hypobarism at a simulated altitude of 3000 m (725 Hpa). The amounts of total glycosaminoglycans (GAG) and hydroxyproline (Hp) were determined in urine portions collected during the period of hypobaric exposure and during the remaining part of the day. Transient shock-like symptoms with high excitement and seizures occurred in 2 of 12 rats during reduction of air pressure for the first time and a significant augmentation of urinary GAG output was found in both rats during this hypobaric period. The remaining rats showed only slight dysbaric signs in the tympanic cavity during ascending to simulated altitude and return to environmental air pressure in the hypobaric chamber. These animals excreted lower amounts of GAG during the hypobaric periods and higher amounts of Hp during the next days after hypobarism.