The results of a study of glyoxaline, pyrrole, and pyrazole by the molecular-orbital method are reported. The theoretical data provide a detailed interpretation of the observed course of electrophilic substitutions in these heterocycles. Unequivocal predictions of the orientations for nucleophilic and radical substitutions in these compounds also emerge, but at present no relevant experimental information is available, with the exception of a possible case of radical substitution in glyoxaline. The present study also indicates that in conjugated systems, the electronegativity of a nitrogen atom carrying a formal negative charge (or a formal negative half-electronic charge) is appreciably less than that of a neutral carbon atom. This should assist in the assignment of electronegativity parameters in future molecular-orbital calculations.