Routine laboratory and lipid peroxidase parameters in the blood of Beagle dogs under 1 year of age (7 males, 7 females) and over 9 years of age (7 males, 7 females) were compared. The mean corpuscular haemoglobin (MCH) and haemoglobin (Hb) values, plasma total protein (TP) and globulin concentrations in the older dogs were significantly higher than in the younger ones by 13%, 6%, 10% and 15%, respectively. The plasma inorganic phosphate concentration, however, was 30% lower in the older dogs than in the young ones. The concentration of malondialdehyde (MDA) was 30% higher, while the glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) activities were around twice as high in the older dogs as in the young ones. However, in the older males the concentration of reduced glutathione (GSH) was 43% and in the older females 9% less than in the young ones. Extremely high levels of MDA and low levels of GSH were only found in the older males. GSH-Px and SOD activities were significantly higher in the older male dogs than in the young ones. The activities of these enzymes were highest in the older females. From our results, we suspect that old dogs, especially males, are particularly exposed to the harmful effects of free radicals and lipid peroxidation.