To clarify the pathophysiological significance of cytokines in chronic pancreatitis (CP), we analyzed tissue expressions of various cytokines in the onset and progression of spontaneous CP in the WBN/Kob rat. Four-week-old male WBN/Kob rats were fed a special pellet diet (MB-3) for 20 weeks, and 6 rats were killed every 4 weeks. Pathologically, CP occurred at 12 weeks and progressed thereafter. The inflammation and fibrosis peaked at 12 and 16 weeks, respectively. By semiquantitative reverse transcription-polymerase chain reaction, the expression of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and interferon (IFN)-gamma mRNAs peaked at 8, 12, and 16 weeks, respectively. Immunohistochemistry showed IL-6 expression in infiltrating inflammatory cells and vascular endothelial cells, whereas TNF-alpha was expressed in both acinar and infiltrating cells. IFN-gamma was localized to acinar, infiltrating and ductal cells, and its expression intensity showed significant correlation with those of fibrosis, type III collagen and alpha-smooth muscle actin. The in situ hybridization results were consistent with the RT-PCR data. These results suggest that tissue expressions of TNF-alpha and IL-6 are involved in the onset of pancreatitis and that IFN-gamma expression is related to the progression of CP.