We investigated whether angiotensin (ANG) II and its receptors contribute to lipopolysaccharide (LPS)-induced microglial activation through activation of the proinflammatory transcription factors nuclear factor κB (NF-κB) and activator protein-1 (AP-1). Using primary microglial cell cultures, we examined whether losartan [ANG type 1 receptor (AT1) antagonist] alters the effects of LPS on: the production of interleukin-1 (IL-1) and nitric oxide, cell morphology, and NF-κB and AP-1 activities. Reverse transcription-polymerase chain reaction revealed that LPS-stimulated microglial cells exhibited marked mRNA expression for AT1, ANG type 2 receptor (AT2) and the ANG II precursor angiotensinogen, whereas non-stimulated microglial cells expressed only those for AT2 and angiotensinogen. We further demonstrated marked peptide/protein expression for AT1 and ANG II in LPS-activated microglial cells. LPS (100 ng/mL)-stimulated microglial cells showed increased concentrations of IL-1 and nitrite (a relatively stable metabolite of nitric oxide), and increased expression of IL-1 mRNA as well as a morphological change from an amoeboid shape to a multipolar (mostly bipolar but sometimes tripolar) rod shape. These effects were all significantly inhibited by losartan treatment (10−5 m or less). NF-κB and AP-1 activities were enhanced in LPS-stimulated microglial cells, effects that were significantly suppressed by losartan (10−5 m). ANG II application enhanced the LPS-induced increases in IL-1 and nitrite concentrations, as well as the LPS-induced morphological changes and AP-1 activation, and these enhancements were inhibited by losartan (10−5 m). These results suggest that endogenous ANG II enhances LPS-induced microglial activities through stimulation of the microglial AT1, which itself evokes activation of the transcription factors NF-κB and AP-1.