We have reported previously that administration of a sublethal low dose of lipopolysaccharide (LPS; 50 microg/kg) prior to the induction of cerulein (Cn) pancreatitis mitigates the pathological course. To clarify the mechanism, we examined apoptosis in the pancreas using the same model. Apoptosis was evaluated by terminal deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling (TUNEL) and transitional electron microscopy. LPS pretreatment at a dose of 50 microg/kg increased remarkably the incidence of acinar cell apoptosis in Cn pancreatitis rats compared with LPS-untreated Cn pancreatitis rats. Apoptosis was observed selectively in acinar cells but was not shown in endocrine cells or ductal epithelial cells. Infiltration of inflammatory cells was scarcely observed. These acinar cells showed the characteristic morphological features of apoptosis by electron microscopy. Administration of LPS at a dose of 50 microg/kg alone caused acinar cell apoptosis but the incidence was much lower than that in the LPS-pretreated Cn pancreatitis rats. The TUNEL labeling was significantly increased depending on the dose of LPS and on the interval between the administration of LPS and that of Cn. These results suggest that the pathological features of acute pancreatitis might be modified by the presence of nonfatal endotoxemia through the induction of acinar cell apoptosis.