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Few studies have been devoted to the interaction between age and stress. However, in view of the age-related changes in various components of the stress responses, the effects of stress may not be constant with age. In this study, we used a dimensional approach to compare open-field behaviour of B6D2F1 female mice, aged 5–6, 11–12, 17–18 and 23–24 months, exposed to a chronic ultramild stress (CUMS) procedure, solely based on nonnociceptive socioenvironmental stressors. Three behavioural dimensions emerged from the principal-component analysis; these were labelled as motor reactivity, exploratory activity, and emotional reactivity. Despite a major effect of age on the three dimensions, we could not conclude that CUMS had any influence as a function the age of the subjects. At all ages, CUMS increased motor activity and had no clear-cut effect on emotional reactivity and exploratory activity. The …