Neprilysin (Nep) is known to be important to degrade $A{eta}$ derived from amyloid precursor protein (APP) by cleavage with $eta-and;gamma$-secretases. In order to determine whether a correspondence between $A{eta}-42/{gamma}-secretase$ activity and Nep levels exists in postnatal aging of transgenic mice expressing either neuron-specific enolase (NSE)-controlled human mutant presenilin-2 (hPS2m) or APPsw alone, the levels of Nep expression and $A{eta}-42/{gamma}-secretase$ activity were examined age of 5, 12, and 20 months, respectively. The levels of Nep expression in both types of transgenic brains were decreased relative to those of control mice in a aging-related manner, while the level of $A{eta}-42/{gamma}-secretase$ activity was reversibly increased. Thus, changes in $A{eta}-42$ may all reflect variation in amounts of Nep enzyme.