Adult onset nesidioblastosis (AON) is an extremely rare entity associated with hypersecretion of insulin. Previous reports have demonstrated that the somatostatin analog, Sandostatin (SMS), will control the clinical symptoms induced by infantile nesidioblastosis. We hypothesized that insulin, C-peptide, and secondary peptide secretion from AON is provocable. We also hypothesized that SMS would suppress both basal and provoked primary and secondary peptide secretion in AON. To test this hypothesis, in a patient with AON, 13 gut peptide levels were determined at set intervals during provocative testing with a test meal, a calcium infusion, a secretin bolus, and a glucagon bolus. These tests were repeated under the influence of SMS. Insulin, C-peptide, and pancreatic polypeptide (PP) levels were elevated in the basal state. SMS suppressed all three peptides (mean 68%) (p less than 0.05). Basal fasting glucose rose by 65%, and glucose ratios were raised throughout all four tests. Insulin:glucose ratios decreased during SMS therapy. Insulin and PP secretion was increased by all four provocative tests (mean 458% and 665% above baseline, respectively). C-peptide was provoked by three tests (mean 204%). Peptides with normal basal values were also provocable. GRP and glucagon were provoked by secretin stimulation (182%, 186%, respectively). Calcium infusion stimulated CIP release by 372%. SMS suppressed the peak provoked peptide levels in all positive provocation tests (p less than 0.05). Peak provoked insulin values were decreased by 59%, C-peptide by 75%, and PP by 92%. Peak provoked glucagon, CRP, neurotensin, and GIP levels were decreased by 20%, 65%, 51%, and 73%, respectively. The patient has been maintained on SMS (25 micrograms bid) for 1 yr and has shown decreased insulin levels, normal glucose levels, and, at 1 yr, leads an asymptomatic normal life. SMS is able to suppress primary and secondary peptide secretion in both the fasting and provoked state. The long-term efficacy of SMS may be predicted by its ability to suppress primary peptide release during peak provocation.