The stability of cis,trans-dihydropyridines and cis,trans-pyrans has been studied using ab initio methods, The strain introduced by the trans double bond has been determined relative to the cis,cis-isomers and introduces 58-69 kcal.mol(-1) of strain energy, at the G3 level of theory, depending on the particular isomer. Double bond rotation barriers have been calculated at the MRMP2/MCSCF level and range from 2.2 kcal.mol(-1) to 11.0 kcal.mol(-1), significantly lower than butadiene (50.3 kcal.mol(-1)). Evidence of resonance through delocalization of the pi electrons is present for the conjugated double bond isomers which lowers the activation barriers. The transition states for trans double bond rotation have significant biradical character but markedly less than that for butadiene. The early transition states with H-C=C-H dihedral angles of 130-150 degrees, as opposed to 90 degrees for butadiene, are consistent with the reduction in the natural orbital occupation numbers. We could not locate a minimum for a structure having both double bonds in the trans configuration and so report that one trans bond is the most the rings can accommodate.