The role of activator protein-1 (AP-1) in inflammation is primarily unknown. AP-1 was evaluated in nuclear extracts from alveolar macrophages and whole lung nuclear extracts during acute lung injury after deposition of IgG immune complexes. AP-1 activation occurred in macrophages and in whole lung extracts, but with distinctly different time courses. Low levels of constitutive AP-1 were observed in normal rat lung as determined by the electrophoretic mobility shift assay. Increased AP-1 was detected 2 hours after initiation of the inflammatory response in lung with a further increase by 4 hours, while AP-1 activation was found in alveolar macrophages 0.5 hour after onset of the inflammatory response. mRNAs and proteins for c-fos, c-jun, jun-B, and jun-D were all up-regulated in whole lung tissues and in alveolar macrophages during acute lung injury induced by IgG immune complex deposition. Depletion of lung macrophages sharply reduced AP-1 activation, as did anti-tumor necrosis factor-alpha, whereas complement depletion showed no effect on lung AP-1 activation. The data suggest that activation of AP-1 occurs in both alveolar macrophages and in the lung, and this activation process is macrophage- and tumor necrosis factor-alpha-dependent.