To clarify the origin of nonplanarity in eight pi -electron heterocyclic conjugated systems, an energy component analysis was carried out for the ground states by using ab initio MCSCF method with 6-31++G(d,p) basis set. Inspection of the energy components comprised in the total energy reveals that the type of pseudo-JT stabilization is classified into two groups, one in which the stability of nonplanar structure arises from a lowering of the interelectronic and internuclear repulsion energies and the other in which the stability results from a lowering of the electron-nuclear attraction energy. This sharp distinction in energy changes is accounted for in terms of an expansion or contraction of the molecular skeleton and a proximity among the nuclei and the electron clouds owing to a folding of the six-membered ring. Besides, it is shown that the theoretical structural characteristics for 1,2-dihydro- and 1,4-dihydro-1,2,4,5-tetrazines are in good agreement with available experimental facts.