Table 1. The activities of glutathione related enzyme systems in red blood cells and plasma levels of muscle originated enzymes before and after the 80-km ride in four endurance horses. Values are means (± SEM). The level of significance is set at p < 0.05, paired samples t-test. Pre After 80 km ride Glutathione peroxidase (μmol·mg-1 Hb-1·min-1) .43 (.06) .53 (.03) Glutathione reductase (nmol·mg-1 Hb-1·min-1) 1.84 (.15) 1.66 (.13) Glutathione-S-transferase(μmol·mg-1Hb-1·min-1) .020(.002) .020 (.002) Creatine kinase (U·l-1) 275 (43) 568 (137) Aspartate aminotransferase (U·l-1) 387 (13) 423 (35)