TO test whether mitogen-activated protein kinases (MAPKs) are involved in microglial activation, pure microglia prepared from 1- to 3-day-old rat brains were activated with either 10 0 ng/ml lipopolysaccharide (LPS) or 5 nM synthetic β-amyloid (Aβ) (25-35). The patterns of MAPK activation following LPS and Aβ treatment were very similar. Three MAPK subtypes, p38, extracellular signal-regulated kinase (ERK) and c-Jun N- terminal kinase/stress-activated protein kinase (JNK/SAPK) were activated within 1 5 min and the activities of p38 and ERK were rapidly reduced to background level within 3 0 min while that of JNK was maintained for over 1 h. Both inhibitors of p38 (SB203580) and ERK pathway (PD098059) reduced LPS-induced nitric oxide (NO) release and A β-induced tumor necrosis factor α-(TNF-α) release. Furthermore, co-treatment of SB203580 and PD098059 additively reduced NO and TNF α-release. These results suggest that MAPK, at least p38 and ERK, mediate LPS-, and Aβ-induced microglial activation.