We investigated static second hyperpolarizabilities (gamma) of 1,4-phenylendiamine (PDA) and 4,4'-diaminodiphenylamine (DADPA) by using the ab initio molecular orbital (MO) methods. The DADPA molecule is assumed to have a planar structure, in which two PDA are linked by a central nitrogen (N) atom, and is considered to be the simplest model for extended dendrimers including linking N atoms. We also examined dicationic PDA and DADPA in order to clarify the effects of charged defects in pi-conjugated systems on gamma values. The gamma value is shown to be enhanced nonlinearly by linking two pi-conjugated units via an N atom. It is also found that the introduction of positive charges causes not only the change in the sign of gamma but also a significant enhancement of the magnitude. Judging from the present results, the extended dendrimers involving linking N atoms are expected to exhibit significantly large gamma values due to an extension of pi-conjugation, and the introduction of positive charges is further predicted to cause drastic changes both in sign and magnitude of their gamma values.