The purpose of this study was to determine whether acute strenuous physical exercise, partially performed under moderate altitude, influences the susceptibility of plasma lipids to peroxidation. Eleven male amateur cyclists took part in a 95-km mountain bike challenge with a cumulative altitude difference of 2340 m. Blood samples were obtained before and immediately after the race in order to determine plasma lipid and lipoprotein concentrations, and to assess the susceptibility of the former to peroxidation. Neither plasma lipid nor lipoprotein levels changed significantly after the race. However, the susceptibility of plasma lipids to peroxidation increased by 71.8%. We concluded that strenuous physical exercise partially performed under hypoxic conditions increases the susceptibility of plasma lipids to peroxidation in untrained subjects.