AIM: To study the feature of pancreatic microcirculatory impairment, especially the initial changes, in caeruleininduced experimental acute pancreatitis (AP). METHODS: The pancreatic microcirculation of caeruleininduced AP model was studied by intravital fluorescence microscopy with FITC-labeled erythrocytes (FITC-RBC), scanning electron microscopy of vascular corrosion casts, and light microscopy of Chinese ink-injected/cleared tissues. RESULTS: Animals in caerulein-treated group showed hyperamylemia (2), pancreatic oedema, infiltration of inflammatory cells in pancreas. Constrictions of intralobular arteriolar sphincters, presence of vacuoles in all layers of sphincter, and gross irregularity in capillary network of acini were found in the AP specimens. The decrease of pancreatic capillary blood flow (0.34 0.10 nl.min-1 vs 0.91 0.06 nl. min-1 of control, P<0.001), reduction of functional capillary density(277 13 cm-1 vs 349 8 cm-1 of control, P<0.001), and irregular intermittent perfusion were observed in caerulein-induced groups. CONCLUSION: Impairment and constriction of pancreatic intralobular arteriolar sphincter are the initial microcirculatory lesions in the early phase of acute pancreatitis, and play a key role in the pancreatic ischaemia and pancreatic microvascular failure in acute pancreatitis. Zhou ZG, Chen YD, Sun W, Chen Z. Pancreatic microcirculatory impairment in experimental acute pancreatitis in rats. World J Gastroenterol 2002; 8(5):933-936 INTRODUCTION Etiopathology of acute pancreatitis (AP) is not fully understood[1-19]. Microcirculatory impairment has long been recognized as one of the etiological factors of acute pancreatitis[20]. Pancreatic microcirculatory disturbance may act as initiating factor or aggravating/continuing factor. However, the mechanism of microcirculatory impairment in acute pancreatitis is complex; there are questions concerning local pancreatic microcirculatory change in acute pancreatitis and the features of pancreatic microcirculatory disturbance in various stages of AP remain subject to further study[21-28]. To investigate the feature of the pancreatic microcirculatory impairment in the early-stage of caerulein-induced experimental acute pancreatitis, dynamic method of microcirculatory research combined with static method had been carried out in this study.