The intramolecular cycloaddition of singlet 2,4-~yclopentadienyIcarbenteo form benzvalene and reactions of related systems have been studied with ab initio molecular orbital theory. These systems were methylene plus butadiene and the intramolecular reactions of allylmethylene, 2cyclopropenykcarbene, 2-cyclobutenylcarbe.ne, and 2-cyclopentenylcarbene. Geometry optimizations were performed with restricted Hartree-Fcck calculations and the 3-21G.basis set. The effect of electron correlation was included with second-order Mdler-Plesset theory with the 6-31G* basis set on the 3-21G optimized geometries. The energies of activation were found to be dependent upon ring strain and spatial orientation of the filled carbene in-plane hybrid orbital during the initial electrophilic phase of the carbene addition. A decrease in activation energy for intramolecular cycloadditions is noted for systems that approach the idealized geometry found with intermolecular additions of carbenes to alkenes.