Background Multiple organ dysfunction syndrome in the elderly (MODSE) is a problem with high mortality in the critical care of elderly patients.The pathogenesis of MODSE remains elusive.This study aimed to establish rat models of MODSE and to investigate the pathogenetic mechanism responsible for the development of MODSE in the rat models.Methods Twenty-four-month old rats (elderly) received intravenous injection of lipopolysaccharide (LPS) to induce rat model of MODSE.In the model,we observed the physical responses,biochemical indices changes,histopathological features of vital organs,including lung,liver,heart,and kidney.We also investigated the sequence of individual organ dysfunction and changes of proinflammatory factors.Three-month-old rats.serving as young rat controls,received parallel procedures.Besides,normal saline injection was also performed on elderly and young control rats.Results All rats displayed different degree of physical response after LPS injection,preceded by deterioration of respiratory status.At 6 hours,lung injury was observed,which started eariier than other organ injury that was observed in about 24 hours.Furthermore,all vital organ injury was more severe in elderiy rats than in young rats at the same time points.After LPS injection,pulmonary alveolar macrophages apoptosis rate increased obviously,and was more significant in elderly rats ((43.4±8.4)％) than in young rats ((24.2±3.0)％).LPS injection also enhanced tumor necrosis factor α (TNF-α) concentration significantly in these organs.Its peak concentration appeared at 6 hours in lung tissue and at 24 hours in other organs after LPS injection.TNF-α level was higher in elderly rats than in young rats at the same time points.The increase was most significant in lung tissue.After intravenous administration of LPS.toll-like receptor 4(TLR4) expression in lung tissue was upregulated markedly,and peaked at 6 hours.In contrast,upregulation of TLR4expression in liver peaked at 24 hours,lagging behind that in the lung.Conclusion Lung is the first and most seriously injured organ in rat model of MODSE and it may play an “initiating” role in the development of MODSE.