Quinidine-like immunoreactivity (Abbott TDx Quinidine fluorescence polarization immunoassay) was measured in blood serum of control rats and rats exposed to hypoxia in a hypobaric chamber (7 500 m for one hour). The mean "quinidine" levels (mean±S.D.) were 0.159±0.058 and 0.260±0.110 («mol quinidinq/1 serum), respectively (p<0.01).