Exposure to chronic hypoxia (CH) results in hypoxic pulmonary hypertension. Two mechanisms are involved: structural remodeling of the vessel wall and vasoconstriction. We centered our interest to changes of pulmonary vascular smooth muscle reactivity in early phase of exposure to CH. We studied isolated pulmonary vessels from adult male rats exposed for 4 days to isobaric hypoxia (FiO2 = 0.1) and compared them with normoxic controls. The bath of the small vessel myograph was saturated by gas mixture containing 21% or 95% of O2 with 5% CO2 and we measured reactions of vessels to PGF2a and acute hypoxic challenge with 0% O2. We didn't observe any difference when the normoxic conditions were setted in bath. When the bath oxygenation increased to 95% O2, the response to hypoxic challenge decreased in rats exposed to hypoxia. It did not change in normoxic controls. We hypothesized that reduced reactivity of vessels from hypoxic rats in hyperoxia results from depletion of antioxidant systems due to increased free radical production during exposure to CH. Supported by GACR 305/06/P385, GACR 305/05/0672 and 1M6798582302.