2-Phenethylbenzimidazole, 2-phenethylbenzoxazole and 2 phenethylbenzothiazole derivatives were synthesized to explore the structural features of the serotonin 5-HT2B receptor antagonists. Those molecules were designed to recognize the 5-HT2B receptor and to discriminate it from the 5-HT2A and 5-HT2C subtypes. All compounds were characterized by binding affinity determination for 5-HT2A and 5-HT2C subtypes and antagonistic activity for 5-HT2B receptor in rat stomach fundus. None of the new compounds showed affinity for 5-HT2A and 5-HT2C subtypes, but some of them displayed antagonistic activity in rat stomach fundus at micromolar concentrations.