The hallmark of severe acute pancreatitis (SAP) is massive acinar cell death by necrosis. However, programmed, apoptotic acinar cell death has also been observed. Little is known about the dynamics, localization, and inductive factors of acinar cell apoptosis in SAP. We therefore induced SAP in rats by retrograde infusion of 3% sodium taurocholate. Starting as early as 5 minutes after taurocholate administration, small scattered groups of acinar cells showed zymogen degranulation, loss of cell polarity, cytoplasmic microvacuolization, and nuclear shrinkage, but no DNA degradation, thus featuring necrosis. The areas of necrotic acini extended at later time points giving rise to larger areas of complete parenchymal breakdown after 6 hours. Parenchymal degradation was paralleled by neutrophil infiltration and significant tumor necrosis factor (TNF)-alpha mRNA up-regulation. Up to the 12-hour interval, apoptotic acinar cells detected by TUNEL were as rare as in healthy pancreata. At 24 hours, however, the acinar apoptotic rate in nonnecrotic parenchyma had dramatically increased. Pretreatment of rats with anti-ICAM-1 antibody prior to pancreatitis induction led to a significant reduction of neutrophil infiltration along with decreased TNF-alpha mRNA expression throughout the 24-hour observation period without affecting the presence and dynamics of necrosis. However, anti-ICAM-1 pretreatment decreased the extent of acinar cell damage by necrosis and extensively suppressed acinar cell apoptosis. We conclude that taurocholate induces two sequential patterns of acinar cell death in terms of very early necrosis followed by late apoptosis during the postacute phase of SAP. The progression of necrosis and the late apoptotic acinar cell death seem to be influenced by the local presence of neutrophils via a TNF-alpha-dependent mechanism. In addition to augmenting necrosis, neutrophils might have an apoptosis-inducing potential in SAP.