The conformational behavior of the cyclohexenone ring in abscisic acid (1) and its analogs (2-5) with a fused cyclopropyl ring was investigated by a low-temperature NMR analysis and computer-aided calculations of the model compounds. The 1 H signals of 1 separated into two sets below 250K, which was a coalescence temperature, in a 99.4:0.6 ratio at 185K, although the small set of signals was not complete. The large and small sets of signals were considered to correspond to an envelope 1-i with the axial side-chain and another envelope 1-ii with the equatorial side-chain. At 185K, the free-energy barrier for the ring inversion between 1-i and 1-ii was determined to be ca. 11kcal/mol, and the free-energy difference between the two conformers was ca. 1.4kcal/mol. The 1 H signals of 2-5 never broadened even at 200K, suggesting that these analogs are more flexible than 1. These experimental results were consistent with those obtained by calculations using models 6-10. Introducing a cyclopropyl group into the ring of 1 lowered the energy barrier for ring inversion and varied the conformational ratio at equilibrium between the minimum-energy conformers.