The insertions of methylene into the CH and CC bonds of ethane and into a CC bond of cyclopropane are calculated by using third-order perturbation theory with the 6-31G(d) basis set. At this level of theory, the barriers for these reactions are predicted to be 0.2, 46.0, and 2.2 kcaljmol, respectively. Thus, the introduction of strain has a dramatic effect on the barrier to insertion into a heavy atom-heavy atom bond.