The 1, 3-dipolar cycloaddition reactivity of pyridine N-oxides to phenyl isocyanate was calculated by the MINDO/3 MO method using the perturbation equation derived by Klopman and Salem. The calculation did not predict the low reactivity of acceptor substituted pyridine N-oxides. On the basis of the calculation data, the general 1, 3-dipolar cycloaddition reactivity of pyridine N-oxides towards various phenyl isocyanates is discussed in terms of the concept of cyclic conjugation. The aromaticity of the pyridine N-oxide may play an important role in determination of the reactivity. In connection with the cycloaddition, the 1, 5-sigmatropic rearrangement of the primary cycloadducts and the pyrolytic reaction behavior of the 2, 3-dihydropyridine derivatives formed by a 1, 5-sigmatropic shift from the primary adducts are discussed on the basis of the MINDO/3 calculation data.