In the ABA receptor (PYR/PYL/RCAR, PYLs)-ABA complex, ABA intrudes deeply into the ligand pocket of PYLs to be tightly packed by the protein residues. In the solvent-excluded molecular surfaces of some complexes, a narrow tunnel is found in the direction of the C3'-H. This is located on a side of the gate which is closed in accordance with formation of the complex. The crystal structure of PYLs-ABA-protein phosphatases 2C (PP2C) ternary complex shows that this tunnel is covered with PP2C. This indicates that the linear hydrophobic alkyl chain at C3' fits into the tunnel with hydrophobic interactions, and if the chain sticks out over the surface of PYLs, it may interfere with the PYLs-PP2C interaction. Based on this speculation, we designed and synthesized 3'-alkylsulfanilated ABA, ASn, where n is a carbon number of an alkyl chain, for finding an antagonist of PYLs. In seed germination and seedling growth assays using lettuce and Arabidopsis thaliana, ASn acted as an ABA mimic when n<4, whereas an inhibitor of exogenous ABA activity when n>4. AS6 exhibited the antagonistic effect on the PP2C assay and the ABA-responsive transgenic plants (MAPKKK18::GUS) assay, whereas the effect of AS2 was agonistic similarly to ABA. Radish seedlings treated with AS6 wilted more rapidly than the non-treated seedlings. These results suggest that the length of an alkyl chain of ASn modulates an interaction between PYLs and PP2C; ASn acts as the agonist when n<4 and the antagonist when n>4.