The changes in the residual function of the exocrine pancreas before and after the operation for chronic pancreatitis were examined with N-benzoyl-L-tyrosyl-p-aminobenzoic acid (NBT-PABA test) in 15 patients. The results of the NBT-PABA test classified by the surgical formula proved that no significant difference was found between the pancreatectomized group (pancreaticoduodenectomy and distal pancreatectomy) and the decompression group (pancreatojejunostomy and transduodenal exploration of the pancreatic duct), and no improvement on the outcome of the postoperative NBT-PABA test, as compared with the outcome of the preoperative test, was noted in both groups. When the state of the fibrosis of the pancreas was classified by the grade for comparison, it was observed that the outcome of the NBT-PABA test of grade 1, in which little fibrosis was made, was better than the outcome of the test of grade 3 before operation (p less than 0.01) and after operation (p less than 0.01). In each of the grades, however, no difference was found between the outcome of the preoperative NBT-PABA test, and that of the postoperative test. It was presumed from these findings that the residual function of the exocrine pancreas after operation was dependent largely on the degree of pancreatic fibrosis at operation.