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One of the main causes of poor quality of frozen–thawed dog sperm is oxidative stress (i.e. higher production of reactive oxygen species not compensated by improved antioxidant protection). This event is known to impair sperm functionality by attacking plasma membrane, acrosome, mitochondria, and DNA. Spermatozoa are particularly susceptible the oxidative stress, mainly due to the reduced cytoplasm and the high content of polyunsaturated fatty acids (PUFA) in the membrane, which allows the spermatozoa to be motile and confers a higher resistance against the damages caused by cryopreservation, but makes the sperm more susceptible to the attack of the reactive oxygen species (ROS). The present study aimed to evaluate the effects of antioxidant supplementation on semen extender (Tris-egg yolk-citrate-glicerol) with glutathione (GSH) and vitamin E on the quality of cryopreserved dog sperm. Ejaculates of 12 dogs were divided in pools of 3 ejaculates with at least 70% of motility. Each pool was diluted with 7 different extenders for treatment groups as follows: control, vitamin E (1, 5, and 10mM), and reduced glutathione (GSH; 1, 5, and 10mM) and submitted to cryopreservation. Samples were thawed (37°C/30′) and evaluated for motility, vigor, percentage of sperm showing intact membrane (eosin/nigrosin), and acrosome (simple stain fast-green and bengal rose), mitochondrial activity (3–3′-diaminobenzidine-DAB), and sperm susceptibility to oxidative stress (TBARS). Statistical analyses were performed using the SAS system for Windows (SAS Institute Inc., Cary, NC, USA; least significant differences test and Spearman correlation; P<0.05). Samples treated with 1mM of GSH showed a higher percentage of sperm with intact membrane when compared with the control (11.21±2.84 and 6.21±1.16%, respectively; P<0.05). On the other hand, treatment with 5mM of GSH showed better results regarding mitochondrial activity. Vitamin E supplementation also played a protective role on mitochondrial activity; samples treated with 1mM showed a lower percentage of DAB III sperm (cells with severely compromised mitochondrial activity) when compared with the control group (5.61±0.7 and 8.62±1.05%, respectively; P<0.05). Both vitamin E and GSH are important non-enzymatic antioxidants responsible for the destruction of the hydroxyl radical. Despite the positive influence of these antioxidants on mitochondrial status, no effect was found on the other variables studied. These results indicate that the action of both antioxidants in dog sperm would be mainly intracellular. Furthermore, other ROS could be responsible for the other damages caused by cryopreservation on the other sperm functionalities (i.e. membrane, acrosome, DNA, oxidative status). Therefore, the use of a combination of enzymatic and non-enzymatic antioxidants could be an alternative to overcome the deleterious influence of oxidative stress in cryopreserved semen of dogs. The authors thank the Brazilian army for the dogs used in this study. [ABSTRACT FROM AUTHOR]