Effects of somatostatin on extrapancreatic glucagon secretion in totally depancreatized dogs were examined. Somatostatin infusion at a rate of 3 microgram/min showed a rapid decrease of total glucagon-like immunoreactive materials (total GLI) measured by nonspecific antiserum, AGS 10, and gut glucagon immunoreactivity (gut GI) measured by specific antiserum, AGS 18, in systemic blood. Gut GLI calculated as the difference between total GLI and GI did not decrease significantly within 30 min. No changes of blood glucose were noted. Significant decreases of all glucagon fractions were observed when the rate of somatostatin infusion was increased to 10 microgram/min and prolonged for 90 min, whereas again blood glucose did not change at all. It is concluded that somatostatin inhibits both gut GI and GLI secretion, although gut GLI remains in circulation longer than gut GI. Suppression of gut GI is not effective for the reduction of blood glucose once an extreme hyperglycemia is brought about by insulin deficiency.