Employing a ligand‐based approach, 3‐methoxyquinoxalin‐2‐carboxamides were designed as serotonin type‐3 (5‐HT3) receptor antagonists and synthesized from the starting material o‐phenylenediamine in a sequence of reactions. The structures of the synthesized compounds were confirmed by spectral data. These carboxamides were investigated for their 5‐HT3 receptor antagonisms in longitudinal muscle myenteric plexus preparations from guinea‐pig ileum against a standard 5‐HT3 agonist, 2‐methy‐5‐HT, and their antagonism activities are expressed as pA2 values. Compounds 6a (pA2: 7.2), 6e (pA2: 7.0), 6f (pA2: 7.5), 6g (pA2: 7.5), 6n (pA2: 7.0), and 6o (pA2: 7.2) exhibited antagonism greater than that of the standard 5‐HT3 antagonist, ondansetron (pA2: 6.9).