Cerebral sterols were examined in Wistar strain rats, subjected 4 h, 24 h, 14 days or 2 months earlier to severe (2% oxygen) and moderate (7% oxygen) experimental hypoxia. From brains of the experimental animals myelin was isolated and examined by gas chromatography coupled with mass spectrometry (GC-MS) to separate and to identify free and esterified (following hydrolysis) cerebral sterols. In both types of hypoxia, the same compounds were identified in fractions of free and esterified sterols. Slight differences in sterol content were noted between the two experimental models, involving cholesterol, the main sterol of the myelin sheath, its smaller sterol precursors (desmosterol, lanosterol) and the less abundant sterols (beta-sitosterol, 24-ethyl-4-cholesten-3-on, cholesta-3,5-dien, cholest-4-en-3-on, 22-propyl-3 beta-hydroxy-5,24-cholestadien). Both types (severe and moderate) of hypoxia were found to induce similar alterations in patterns of cerebral sterols 24 h, 14 days and 2 months following the exposure. Quantitative differences in the sterol pattern were detected only at the very early stage (4 h) of the experiment.