Abstract The 1,3-dipolar cycloadditions of phenylglyoxylonitrile oxide, benzonitrile-N-phenylimine, or N-phenyl-C-p-nitrophenylnitrone to norbornadiene, 2,3-dicyanonorbornadiene, and 2,3-bis(methoxycarbonyl)norbornadiene give the endo-adducts, together with the exo-adducts. These findings show that the 1,3-dipolar cycloadditions to norbornadienes do not follow the “exo rule.” It is suggested that the present 1,3-dipolar cycloadditions are kinetically controlled reactions. The endo-side of norbornadienes is found to be homoconjugated by their NMR spectra. The homoconjugation must be responsible for the observed phenomenon.