Ab initio molecular orbital calculations have been used to model the stereochemistry of bicyclo[2.1.0]pentane hydroxylation. Equilibrium geometries and transition states were fully optimized at the MP2 level of theory using the 6-31G* and 6-31G** basis sets; all transition states were confirmed to be first-order saddle points by MP2 frequency calculations; energy differences and barrier heights were computed at the MP4 level with and without spin projection. Both the endo- and exo-bicyclo[2.1.0]pent-2-yl radicals are significantly pyramidal, but are nearly equal in energy (Delta E < 0.3 kcal/mol) and are separated by a very low (<0.4 kcal/mol) barrier. The barrier for trapping the bicyclopentyl radical by H2S is 1.5 kcal/mol lower for the endo radical. Even though the endo and exo bond strengths are nearly identical in bicyclo[2.1.0]pentane, abstraction of the endo hydrogen via the OH radical is favored over the exo hydrogen by 1.4 kcal/mol. Concerted oxygen insertion was modeled by reaction of bicyclo[2.1.0]pentane with water oxide, H2OO; the insertion transition state yielding the endo alcohol is 1.3 kcal/mol lower in energy. The endo preference of all the reactions in the present study can be attributed to cyclopropylcarbinyl stabilization of the transition states. The relevance of these calculations to cytochrome P-450 hydroxylation is discussed.