6-7-month-old male Wistar rast were divided into three groups. On group of rats remained intact and lived under normal conditions (Group 1; n = 12). Groups 2 (n = 12) and 3 (n = 14) were exposed in hypobaric chamber (18-20 h a day and 2 h a day, respectively) for 2 weeks. The partial O2 pressure was maintained equivalent to 5000 m altitude. The hypoxia produced decreases of structural vascular resistance in hindquarters in Group 2 by 20-25% and in Group 3 by 17-20% (p < 0.05) under all perfusion pressure values. No significant changes in capillary density, capillary per fibre ratio in m. extensor hallucis proprius were found in rats exposed to hypobaric hypoxia for both durations. The cross-sectional area type I fibres (but not II fibres) (CSA) in Group 2 was significantly (p < 0.05) decreased (611 +/- 44 microns 2 vs 779 +/- 49 microns 2 in controls), continuous hypoxia produces significant reduction in type I fibre diffusion distances.