The objective of this study was to assess the effect of vitamin E and selenium (Se) supplementation on the blood plasma levels of F2-isoprostanes (F2I) as a marker of oxidative stress in horses in their training period. The horses of the experimental group (E) received supplements of vitamin E and of sodium selenite. It can be stated that supplementation of the diet used with Se and vitamin E caused a non-significant decrease of F2I concentration in the blood plasma only, and a significant increase of plasma concentrations of these antioxidants. The variation of F2I levels probably reflected rather the individual responses of the horses? organisms to the training workload.