In rodents, exposure to chronic mild stress (CMS) is known to induce unresponsiveness to environmental stimuli, as well as sleep disturbances, suggesting some analogies between this syndrome and human depression. Furthermore, numerous studies reported a decrease in nocturnal melatonin concentration in depressed patients, compared with controls. The present study was conducted to test a possible preventative action of daily treatment with melatonin on behavioural alterations induced in C3H/He mice by CMS exposure. In addition to daily spontaneous locomotor activity and preference for sucrose solution, the emotional behaviour of mice was examined in a stressful situation (light/dark choice test), as well as in a situation devoid of constraining components (free-exploratory paradigm), after three weeks of CMS. The results showed that the behaviour of C3H/He mice was disrupted after CMS. Stressed mice exhibited blunted emotional reactivity in both the light/dark choice test and the free-exploratory situation. While unstressed mice presented no variation in their preference for a sucrose solution, stressed mice presented a decrease in such preference towards the end of the CMS exposure. Furthermore, daily spontaneous locomotor activity of the mice was reduced after CMS. Daily treatment of stressed mice with melatonin was able to prevent several CMS-induced disturbances, except in the light/dark choice test, where melatonin was ineffective. Compared to the effects of 10 mg/kg of fluoxetine, which completely prevented CMS-induced dysregulation of behaviour, melatonin was less effective. The present results support the idea that melatonin may be implicated in an homeostatic system which protects animals from disruptions induced by chronic stress.