Disturbances in oxidative metabolism are involved in many acute and chronic diseases, as well as in several other conditions. The objective of the present study was to examine the relationship between the level of intracellular reactive oxygen species in the peripheral blood granulocytes of mice, as evaluated by 2',7'-dichlorofluorescin diacetate (DCFH-DA), a sensor of reactive oxygen species, and the aggressive behavior of these mice, as estimated by the resident/intruder test. Our results showed a significant, linear and positive relationship (P < 0.001) between the intracellular redox status of peripheral blood granulocytes and the aggressive behavior levels of adult male mice (correlation coefficients (R²) ranging from 0.75 to 0.77). This suggests that the granulocytes of aggressively behaving mice have high levels of oxidative stress.