Diesel exhaust particles (DEPs) at three concentrations (5, 35, and 50 mg/kg body weight) were instilled into rats intratracheally. We studied gene expression at 1, 7, and 30 days postexposure in cells obtained by bronchoalveolar lavage (BAL) and in lung tissue. Using real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we measured the mRNA levels of eight genes [interleukin (IL)-1beta, IL-6, IL-10, iNOS (inducible nitric oxide synthase), MCP-1 (monocyte chemoattractant protein-1), MIP-2 (macrophage inflammatory protein-2), TGF-beta1 (transforming growth factor-beta1), and TNF-alpha (tumor necrosis factor-alpha )] in BAL cells and four genes [IL-6, ICAM-1 (intercellular adhesion molecule-1), GM-CSF (granulocyte/macrophage-colony stimulating factor), and RANTES (regulated upon activation normal T cell expressed and secreted)] in lung tissue. In BAL cells on day 1, high-dose exposure induced a significant up-regulation of IL-1beta, iNOS, MCP-1, and MIP-2 but no change in IL-6, IL-10, TGF-beta1, and TNF-alpha mRNA levels. There was no change in the mRNA levels of IL-6, RANTES, ICAM-1, and GM-CSF in lung tissue. Nitric oxide production and levels of MCP-1 and MIP-2 were increased in the 24-hr culture media of alveolar macrophages (AMs) obtained on day 1. IL-6, MCP-1, and MIP-2 levels were also elevated in the BAL fluid. BAL fluid also showed increases in albumin and lactate dehydrogenase. The cellular content in BAL fluid increased at all doses and at all time periods, mainly due to an increase in polymorphonuclear leukocytes. In vitro studies in AMs and cultured lung fibroblasts showed that lung fibroblasts are a significant source of IL-6 and MCP-1 in the lung.