The aim of this study was to examine effect of extruded fodder on the energy metabolism of horses under training conditions. Selected biochemical parameters of energy and protein metabolism parameters were monitored at rest after exercise at the beginning and at the end of the ten-week experimental period. The ten weeks of the training period resulted in a significant decrease in serum glucose concentration, serum total proteins, and plasma creatinine concentration, and a significant increase in plasma urea concentration and haemoglobin in both groups of horses at rest. Post exercise a significant increase in blood urea levels and a decrease in total proteins in both groups demonstrated that the proteins became sources of energy. A significant increase in the haemoglobin level together with a significant decrease in the creatinine level at the end of the experiment indicated an increased aerobic metabolism. Nevertheless, extruded fodder in the horses’ diet did not have the anticipated effect.