Four infants (three boys and one girl, ages 12-89 days) with persistent hyperinsulinism secondary to nesidioblastosis (two) or microadenoma of the pancreas (two) were treated with cyclic somatostatin (S) as part of the preoperative management until subtotal pancreatectomy was performed within 12 to 35 days. The individual dose dependent response of glucoregulatory hormones to exogenous was evaluated by means of the "somatostatin sensitivity test" (SST). Thereby S was infused in stepwise increasing doses, as dictated by the prevailing blood glucose levels, until normoglycemia was achieved concomitantly with a dextrose infusion at rates of 5 mg/kg/min. This procedure resulted only in a partial suppression of insulin, C-peptide, glucagon, HGH and cortisol, without recurrence of hypoglycemia. Compared to baseline levels, plasma concentrations of insulin decreased by 61%, of C-peptide by 64% and a rise of glucagon by 23% was observed. The SST which can be performed under routine clinical conditions, is a useful procedure for evaluating the individual S-dose necessary to achieve normoglycemia. The risk of total S-induced suppression of hormones, such as IRI, IRCP, HGH, glucagon and cortisol can be omitted.