We investigated the impact of blood glucose normalization on plasma levels of somatostatin-like immunoreactivity (SLI) in subjects with C-peptide-negative insulin-dependent diabetes mellitus (IDDM) and in totally pancreatectomized patients. Patients were studied during hyperglycemia and during normoglycemia, which was attained by Biostator-directed feedback insulin infusion. The experiments were performed in the fasted state and after a standardized breakfast. In IDDM (n = 6), basal levels of SLI were significantly higher than in nondiabetic subjects (n = 18). In IDDM, normalization of hyperglycemia was followed by a 40% decline in basal SLI (P less than .05). After the meal, SLI increased to the same absolute levels with or without feedback insulin treatment; however, the incremental response was 60% higher during feedback insulin treatment (P less than .05). Treatment also suppressed fasting and postprandial levels of glucagon, whereas gastric inhibitory polypeptide (GIP) levels were unaltered. In four pancreatectomized patients, normoglycemia tended to lower plasma levels of SLI by 50% (P less than .1). After breakfast, an SLI response was noted during normoglycemia, whereas no significant effect of the meal was seen during hyperglycemia. We conclude that in IDDM and in totally pancreatectomized patients, administration of insulin with subsequent normalization of blood glucose is accompanied by a decline in plasma levels of SLI in the fasted state, whereas the apparent response to a meal is enhanced. These effects on plasma levels of SLI probably reflect to a major extent release of somatostatin from the gastrointestinal tract.