Various azaazulene derivatives were synthesized and their antiallergic activity was examined. The structure-activity relationship among various derivatives modified by introducing substituents at the 1-, 2-, or 3-position of the azaazulene ring was investigated.The inhibitory activities on allergic histamine release of the compounds bearing a 5-tetrazolyl group at the 3-position were more potent than those of the corresponding compounds with other groups (CN, COOH, and CHO). The compounds substituted with amino, azide and carboxymethylamino groups at the 2-position showed strong inhibitory activity. The compounds with various phenylalkyl groups at the 1-position showed a greater activity than those with other substituents. Among the compounds with substituents at the 1-, 2-, or 3-position of the azaazulene ring, 1-benzyl-7-isopropyl-3-(5-tetrazolyl)-1-azaazulen-2-one (18f) and 1-(4-fluorobenzyl)-7-isopropyl-3-(5-tetrazolyl)-1-azaazulen-2-one (19c) had the most potent inhibitory activities on histamine release from mast cells and on passive cutaneous anaphylaxis (PCA) in rats after oral administration (ED₅₀=0.56 and 0.58 mg/kg, respectively).