ab initio molecular orbitaI calculation is performed to estimate the substituent effects for meta-substituted pyridines. Electrostatic potentials are obtained from ab initio wavefunctions of the optimized structures for the meta-substituted pyridines. Electrostatic potenhals are shown to be minimum at nitrogen atom of pyridines. The potenttial minima are good correlated with the substituent constants, ${sigma}_m$ and with the ${Delta}pK_a$, respectively. It is found that the electrostatic potential minima can be used as a useful measure of substituent effects.