The study was conducted on 72 young (3 months) male Wistar rats in spring (March-April) and autumn (October-November).Each experiment lasted 28 days. Animals of the experimental groups were breathing the gas mixture (GM, PO2 = 90 mmHg, the equivalent of 12volume per cent in nitrogen) in the mode 15 min deoxygenation/15 min reoxygenation during two hours.The other group of animals, exceptGM, was orally administered 1 ml of an aqueous suspension of melatonin at pharmacological dose of 5 mg/kg of body massat 10.00 a.m. In spring, combined effect of dosed hypoxia (DH) and pharmacological doses of melatonin reduced the activity of alkaline phosphatase (ALP), but did not affect the activity of acid (ACP) and tartrateresistant acid phosphatase (TRACP).This effect has boosted the concentration of glycosaminoglycans (GAG), which may cause a slowdown in bone remodeling of young rats. In spring, changes in the concentration of amino acids (AA)which are directly involved in the synthesis of bone collagen of young animals after exposure to DH or combined effect of DH and exogenous melatonin at pharmacological doses were unidirectional. In most cases, the concentration of AA significantly reduced, which can be the reason of collagen synthesis inhibition in organic matrix of bone tissue (BT).In autumn osteoblast activity in young rats after exposure to DH and after its joint action with exogenous melatonin in pharmacological doses unchanged. However, activity of lysosomal enzymes after the influence of mentioned factors increased. At the same time, concentration of GAG increased. It may be the outcome of communication between GAG and collagen fibers after exposure to DH, ie destructive changes of the organic matrix of bone. In autumn the increase of total and free cholesterol (TC and FC respectively) concentrations in BT ofratsafterexposure as to DH, so to its jointaction with melatonin, couldbe considered as a sign of possibleseasonal intensification of bone mineralization.
