Ab initio RHF SCF geometry optimizations and MP2 limited geometry optimizations are used to investigate the mechanism of inner-hydrogen migration in free base porphyrin. Previous approximate SCF results using AM1, MNDO, and PM3 methods predict the existence of a highly stable cis isomer close in energy to the most stable trans form and that trans reversible arrow trans interconversion proceeds in an asynchronous two-step fashion via this intermediate. However, the calculated activation energies are very much greater than those observed. Here, at the ab initio MP2 level of correlation, it is found that all (classical) barriers are substantially reduced in height, becoming compatible with experiment : the trans to cis activation energy is 16.7 kcal/mol, the trans to trans saddle energy is 19.3 kcal/ mol, and the relative energy of the cis isomer is 10 kcal/mol. Hence, an asynchronous path remains preferred. The reaction coordinate at small displacements is seen to correlate with the pyrrolic hindered rotation nu(35), observed at 109 cm(-1); for use in one-dimensional models, we find that the involvement of NH bending and stretching motions in the reaction at large displacements results in a globally optimized effective reactant reaction coordinate frequency of 600 cm(-1). Using PM3, zero-point energy considerations are shown to account for just over half of the observed inner-hydrogen isotope effect and to lower the classical activation energy by ca. 5 kcal/mol. The net activation energy is thus estimated to be 12 kcal/mol, no doubt fortuituously close to the value of 13 kcal/mol which has been deduced from experiment using simple tunneling theories.