It has been shown that glucocorticoids can modulate oxytocin (OT) secretion and disrupt maternal behaviour. Because the CB1 receptor (CB1R) has been implicated in some rapid glucocorticoid-induced actions, the present study aimed to evaluate the possible involvement of CB1Rs in maternal behaviour and neuronal activation during lactation. For this purpose, lactating female rats were pre-treated with dexamethasone (DEX) or saline, followed by treatment with AM251, a CB1R antagonist, or vehicle 90 min later. All of the experiments were performed 30 min after the administration of AM251 or vehicle. To evaluate maternal behaviour, the pups were returned to their home cages to the side of the cage opposite the previous nest after 12 h of separation and were filmed for the next 30 min. Aggressive behaviour was evaluated for 10 min following the placement of a male rat in the home cage. For the evaluation of behavioural performance, lactating rats were subjected to a T-maze and open-field tests. The amount of weight gained by the pups was evaluated 15 min after the onset of suckling to determine the amount of milk that they had obtained from the dam. In the central nervous system of lactating rats, c-Fos-positive nuclei were counted in the medial preoptic area, in both the ventral (v) and dorsal (d) parts of the median preoptic nucleus and in the bed nucleus of the stria terminalis (BNST). The number of neurons that were double-labelled for c-Fos/OT was counted in the medial magnocellular subdivision of the paraventricular nucleus, in the periventricular hypothalamic nucleus and in the supraoptic nucleus of the lactating rats. The results show that DEX had the following effects: (1) decreased the amount time the dam spent licking the pups, the amount of time the dam spent in an arched-nursing position and full maternal behaviour; (2) increased the latency to the first attack and decreased front attacks; (3) increased anxiety-like behaviour; and (4) decreased weight gain in the pups. In addition, DEX decreased neuronal activation in all of the investigated hypothalamic and forebrain areas. AM251 administration reversed these parameters, indicating that the behavioural effects and neuronal responses produced by DEX in lactating rats are likely to be mediated by CB1Rs.