Alkenyl-substituted diaryl-nitrile-imines - generated by photolysis or thermolysis of alkenyl-substituted 2,5-diaryl-tetrazoles - undergo a regioselective intramolecular [2+3] cycloaddition to yield new heterocyclic compounds, e.g. fused 2-pyrazolines. With alkinyl derivatives, the corresponding pyrazoles have been formed. UV evidence is given for the intermediate nitrile-imine at - 190°. The latter can be trapped using an excess of carboxylic acid (UV evidence for a new intermediate at - 120°). In this case, the corresponding rearranged addition product N'-acyl-N'-aryl-benzohydrazide is isolated in good yield.