Epimeric 2-methyl-5-phenyl-5-carbethoxy-2-azabicyclo[2.2.1]heptanes were elaborated from N,O,O-tritosylhydroxyprolinol, and the stereochemistry was determined from physicochemical studies. The benzoquinoneinduced writhing test indicated the endo-phenyl epimer to be twice as potent as meperidine and six times more potent than the exo isomer. In terms of brain concentrations, however, the endo-exo potency ratio is 3.7. Evidence is presented which quantitatively relates difference in brain levels between the epimers to their partition coefficients. The large difference in geometry between the exo and endo epimers suggests that their comparable activities are due to differing modes of interaction with analgetic receptors.