The regional activity of phosphate-activated glutaminase (PAG) was assessed in 8-month-old and 28-month-old male Fischer-344 (F344) rats from the temporal cortex (TCX), striatum (STR) and hippocampus (HIPP). Basal activity, as represented by activity in the presence of 10 mM phosphate, was decreased 23% in the TCX from aged rats compared to adults while neither the STR or HIPP exhibited age-related changes. In the presence of 100 mM phosphate, which will maximally stimulate PAG activity, none of the regions displayed age-related changes although all groups showed increased PAG activity of 112-169% compared to corresponding values in the presence of 10 mM phosphate. Kinetic analysis revealed no alterations in the apparent affinity (Km) of PAG for glutamine in the TCX and STR, but the Km value was increased 32% in the HIPP(255±12 μM compared to 336±22 μM). There were no significant changes in Vmax values. Regional variability seen in PAG basal activity and kinetics may reflect regional differences in glutamatergic innervation or glutamate-specific metabolic requirements of these three regions. These possibilities are discussed with regard to the aging process and neural degeneration which may occur as a result of a dysfunctional glutamatergic system.