Anxiety has been implicated in a number of psychiatric disorders, such as generalized anxiety disorder, depression, panic attacks, phobias, obsessive-compulsive disorders and posttraumatic stress disorders. In this study, we investigated the impact of high anxiety on the levels of intracellular reactive oxygen species in lymphocytes, granulocytes and monocytes from the peripheral blood of mice by using a 2',7'-dichlorofluorescein diacetate (DCFH-DA) probe. The behavioural light/dark choice test was used to distinguish highly anxious from less anxious mice. Our results showed that a high anxiety level significantly increased the generation of reactive oxygen species in the peripheral blood lymphocytes, granulocytes and monocytes.