The Klopman-Peradejordi-Gomez method relates the variation of a biological activity, measured in vivo or in vitro, with the variation of the numerical values of a set of local atomic reactivity indices (LARIs). The introduction of local atomic reactivity indices within the Hartree-Fock scheme plus the orientational parameter of the substituents has produced excellent relationships between the electronic structure and the biological activity for many different molecules and biological measurements. Here we present a detailed analysis of how to modify the numerical values of the LARIs in order to increase the biological activity. The concept of local molecular orbital is central to this analysis.