To investigate the mode of inheritance of pancreatic lesions in male WBN/Kob rats, WBN/Kob rats were crossed with DA rats. Pancreata from male rats of backcross generation between Fl and WBN/Kob rats were examined histologically at 12 months of age. In 46 out of 92 (50.0 %) backcross generations, inflammatory cell infiltration and/or pancreatic fibrosis with deposition of hemosiderin, characteristic histological features of WBN/Kob strain, were observed. Namely, the presence/absence ratio of pancreatic lesions was nearly 1: 1 in the backcross generation. However, in another 4 out of 12 (33.3 %) in the Fl generation, pancreatic lesions were observed. From the data of back crossing we may conclude that one recessive gene is responsible for expression of pancreatic fibrosis in male WBN/Kob rats. As for the occurrenece of pancreatic lesions in the Fl generation some modification of recessive type of inheritance is necessary. Further investigation is necessary to clarify this phenomenon.