Reactive oxygen and nitrogen species are important parts of cellular immune response involved in killing intracellular parasites. In order to determine if intracellular parasite infection may be associated with oxidative damage, circulating lipid peroxidation, protein oxidation and antioxidant status were investigated in horses naturally infected with Theileria equi. For that, the T. equi infection was confirmed in 9 horses using an indirect fluorescence antibody test and malondialdehyde (MDA), protein carbonyl (PCO), nitric oxide metabolites (NOx) and reduced glutathione (GSH) concentrations as well as the total antioxidant activity (AOA) were measured in sera from T. equi infected horses and from healthy controls (n = 9). Whereas GSH concentrations and serum AOA were significantly depressed in the infected horses compared to the controls, the serum MDA, PCO and NOx concentrations were markedly increased. These results show the occurrence of oxidative stress amplified by the antioxidant depletion probably due to overproduction of free radicals by activated inflammatory cells in horses naturally infected with T. equi.