The exact resectability of the pancreas without producing diabetes was investigated in dogs, with consideration of pancreatic regeneration. Within the first week after major pancreatectomy, acinar cell division occurred, followed by hypertrophy; these changes resulted in an increase in weight of the remnant pancreas. The regeneration rate correlated well with the resection rate, glucose tolerance, and the insulin secretion of the remnant pancreas. Immediately after resection of more than 92% of the pancreas, severe diabetes developed, and the regeneration rate was 29.9 +/- 6.03% three to six weeks after surgery. Twelve weeks after resection of 74-92% of the pancreas, so-called Sandmeyer's diabetes had developed. The regeneration rate was 45.3 +/- 4.22% in the non-diabetic group, but in the diabetic group it proved to be only 15.4 +/- 2.39%. When less than 74% of the pancreas was resected, diabetes did not occur throughout the postoperative observation periods of 3-18 weeks, and the regeneration rate was 5.5 +/- 6.62%. These results suggested that 74% of the pancreas may be removed without producing diabetes.