We studied the influence of normobaric hyperoxia on bone metabolism in 3 - and 12-month-old male Wistar rats. Maintaining young rats (3 months) during the 14-hour daily sessions under normobaric hyperoxia (90% O2) was accompa­nied by a significant decrease in the concentration of C-termi-nal propeptides of collagen type I (by 36%), the acid phos-phatase activity (by 32%), an increased activity of alkaline phosphatase (by 64%), an increased concentration of pyridinoline (by 37%) and b-CrossLaps (by 8%), glycosa-minoglycans (by 72%) in the blood serum. We believe that normobaric hyperoxia (90%) is accompanied by the distur­bance of collagen synthesis. In adult rats under the same con­ditions for 14 sessions, the concentration of glycosaminoglicans significantly increased by 60% relative to the control values. After 28 sessions of breathing the normobaric gas mixture containing 90% O 2 this parameter increased by 195%. Breath­ing normobaric gas mixture containing both 40% and 90% of O2 for 14 and 28 days decreased the acid phosphatase activity and the tartrat-resistant acid phosphatase activity by 18-25%. We believe that in adult animals 90% hyperoxia does not affect the activity of osteoblasts, but breaks the link between glycosaminoglicans and collagen fibrils, decreases the activity of lysosomal enzymes which are produced by osteoclasts and which can inhibit osteogenesis.