The hypothesis that the expression of protease-activated receptors (PARs) protein is regulated at the level of transcription and that PAR isoforms, PAR-1, PAR-2, PAR-3, and PAR-4, in lung tissue show different patterns of expression in lipopolysaccharide (LPS)-induced acute lung injury (ALI) was tested. Male Wistar rats were rendered endotoxemic by intra-peritoneal injection of LPS (15 mg/kg body weight). We examined the expression of protein and mRNA and the immunohistochemical localization of PAR isoforms in lung tissues 1, 3, 6, and 10 h after LPS administration. Induction of ALI by LPS was confirmed based on histopathological changes. LPS administration induced significant increases in the expression of PAR isoforms (protein) at the level of transcription in ALI. While the time course of PAR-1 and -2 expressions were different, those of PAR-3 and -4 were almost similar. An immunohistochemical analysis showed localization of PAR isoforms in the vascular endothelium, alveolar epithelium, and alveolar macrophages. However, the cellular distribution patterns of PAR isoforms were different. We conclude that LPS induces increase in protein expression of PAR isoforms at the level of transcription in rats with ALI. The differential expression patterns (over a time course) and distribution of PAR isoforms suggests a distinct role for each isoform in the pathogenesis of LPS-induced ALI.