AbstractDopamine and serotonin (5‐HT) in the nucleus accumbens (ACC) and ventral tegmental area of the mesoaccumbens reward pathways have been implicated in the mechanisms underlying development of alcohol dependence. We used a C57BL/6J mouse model with increased voluntary alcohol‐drinking behavior by exposing the mice to alcohol vapor for 20 consecutive days. In the alcohol‐exposed mice, the expression of 5‐HT2Creceptor mRNA increased in the ACC, caudate nucleus and putamen, dorsal raphe nucleus (DRN), hippocampus and lateral hypothalamus, while the protein level of 5‐HT2Creceptor significantly increased in the ACC. The expression of 5‐HT7receptor mRNA increased in the ACC and DRN. Contents of 5‐HT decreased in the ACC shell (ACCS) and DRN of the alcohol‐exposed mice. The basal extracellular releases of dopamine (DA) and 5‐HT in the ACCSincreased more in the alcohol‐exposed mice than in alcohol‐naïve mice. The magnitude of the alcohol‐induced ACCSDA and 5‐HT release in the alcohol‐exposed mice was increased compared with the control mice. Intraperitoneal (i.p.) administration or local injection into ACCSof the 5‐HT2Creceptor antagonist, SB‐242084, suppressed voluntary alcohol‐drinking behavior in the alcohol‐exposed mice. But the i.p. administration of the 5‐HT7receptor antagonist, SB‐258719, did not have significant effects on alcohol‐drinking behavior in the alcohol‐exposed mice. The effects of the 5‐HT2Creceptor antagonist were not observed in the air‐exposed control mice. These results suggest that adaptations of the 5‐HT system, especially the upregulation of 5‐HT2Creceptors in the ACCS, are involved in the development of enhanced voluntary alcohol‐drinking behavior.