The increase in both structural variety and potential industrial applications of pyridine and its derivatives calls for an improved understanding of the interactions of compounds in the class with biological targets. The reactivity of the heterocyclic nitrogen is a primary determinant of the chemical properties of compounds in the class; it also determines the spectrum of their potential metabolites such as the N-oxides. Comparative studies on the reactivity of the nitrogen were carried out for pyridine and its three monosubstituted derivatives 2-aminopyridine (2-AP), 3-aminopyridine (3-AP), and 4-aminopyridine (4-AP) to reveal the structural basis for the differences in their susceptibility to N-oxidation. Molecular orbital calculations were performed to obtain the wave functions for the calculation of the molecular electrostatic potentials (MEP) generated by the molecules. The comparison of the reactivity of the cyclic nitrogen, evaluated from the depth and accessibility of the minimum in the MEP, indicates that the nitrogen in 4-AP will be most susceptible to protonation and will be the most protected from N-oxidation at physiological pH values. The MEP map for 2-AP reveals the smallest minimum in the series of compounds and a considerable reduction in the accessibility of the region near the cyclic nitrogen caused by the proximal substitution. On this basis, 3-AP becomes the most likely derivative to form the ring N-oxide.