Compelling evidence has documented the anxiolytic and mood-enhancing properties of cannabis. In susceptible users, however, consumption of this drug is conducive to panic, paranoia and dysphoria. We hypothesized that the up-regulation of CB1 receptors (CB1Rs) in select brain regions may enhance the vulnerability to cannabinoid-induced anxiety. To test this possibility, we assessed the behavioral impact of a potent cannabinoid agonist (CP55,940; 0.05–0.1 mg/kg, IP) on C57BL/6 male mice, respectively subjected to a prolonged pre-treatment of either the selective CB1R antagonist/inverse agonist AM251 (1 mg/kg/day IP, for 21 days, followed by a 3-day clearance period before testing) or its vehicle (VEH1). Anxiety-like responses were studied in the novel open field, elevated plus maze (EPM) and social interaction assays. While CP55,940 induced anxiolytic-like effects in the EPM in VEH1-exposed animals, it elicited opposite actions in AM251-exposed mice. In this last group, CP55,940 also reduced rearing and social interaction in comparison to its vehicle (VEH2). The divergent effects of CP55,940 in AM251- and VEH1-pretreated animals were confirmed in 129SvEv mice. Immunoblotting analyses on brain samples of C57BL/6 mice revealed that AM251 pre-treatment caused a significant up-regulation of CB1R expression in the prefrontal cortex and striatum, but also a down-regulation of these receptors in the hippocampus and midbrain. Notably, CB1R levels in the prefrontal cortex were negatively correlated with anxiolysis-related indices in the EPM; furthermore, midbrain CB1R expression was positively correlated with the total duration of social interaction. These results suggest that regional variations in brain CB1R expression may differentially condition the behavioral effects of cannabinoids with respect to anxiety-related responses.