To elucidate the nature of a pseudo-Jahn-Teller (JT) distortion from planarity, ab initio MCSCF calculations with the 6-31G(d,p) basis sets were carried out in the ground states of seven- and eight-membered unsaturated heterocycles with eight pi electrons. The monocyclic systems examined were found to show a variety of structural changes. Azepine and oxepin undergo the pseudo-JT distortions from a planar C-2nu to a boat C-S conformation. In 1,5-diazocine, the pseudo-JT distortion takes place in two stages, the initial step being from a symmetric planar D-2h to a skew C-2h structure and the subsequent step from the skew C-2h to a tub C-2 structure. The 1,3,5,7-tetrazocine molecule undergoes pseudo-JT distortions from a symmetric planar D-4h to skew C-4h and crown-like D-2d structures through the respective in-plane and out-of-plane nuclear deformations. Moreover, the C-4h and D-2d structures are distorted into the same tub-shaped S-4 conformation. An inspection of the energy components comprised in the total energy reveals that the stability of nonplanar structures arises from a lowering of the electron-nuclear attraction energy. The energy variation is accounted for in terms of an electrostatic interaction and the proximity among the nuclei and electron clouds owing to a folding of the molecular skeleton. It is shown that the theoretical structural characteristics for azepine, oxepin, 1,5-diazocine, and 1,3,5,7-tetrazocine are in good agreement with available experimental facts.