Mitogen-activated protein kinases (MAPKs) play important roles in cell proliferation, differentiation, and apoptosis. Important functional roles for MAPKs in postmitotic cells have recently been suggested. In the present study, we investigated the effect of aging on the brain ERK (extracellular signal-regulated kinase) and p38 MAPK signaling pathways of Fischer 344 rats. The results show that basal tyrosine-phosphorylated ERK1/ERK2 in cortex of 24-month-old rats was reduced by 36%-59%, compared to 6- and 12-month-old rats (p<.05, 24- vs. 12- or 6-month-old rats). Similarly, the phosphotransferase activities of ERK and p38 MAPK, measured by in vitro immunocomplex kinase assays using myelin basic protein (MBP) as substrate, were shown to be reduced approximately 50% and 59% respectively, in the cerebrocortex of 24-month-old rats (p<.01, 24- vs. 12- or 6-month-old rats). The reductions in basal ERK and p38 MAPK activities are not due to altered protein levels of these kinases as assessed by Western analysis. Immunohistochemically, no age-related differences in ERK expression and cellular distribution were observed However, cytosolic ERK tended to aggregate in brain neurons of aged rats. In contrast brain tyrosine-phosphorylated PLCgamma1 did not change with age. Activation of ERK in response to EGF or PMA was also reduced in cortical brain slices of 24-month-old rats. These results demonstrate an age-associated selective impairment in the MAPK signaling pathways. Moreover, lifelong caloric restriction completely prevented the age-related decrease in basal brain ERK activity and diminished the age-related reduction of p38 MAPK activity. Taken together, these data indicate that ERK and p38 MAPK signaling pathways are impaired in the aged brain and that lifelong caloric restriction modulates these defects in brain intracellular signaling pathways.