The significance of the minimal secretory capacity of pancreatic beta-cells for the stability of the plasma glucose level was studied in 20 patients with insulin-dependent diabetes mellitus. Changes in plasma concentrations of major counterregulatory hormones in response to hypoglycemia were also investigated in these patients to clarify their contribution to diabetic brittleness. beta-Cell function was evaluated on the basis of elevation of plasma C-peptide immunoreactivity (CPR) during the intravenous glucagon test with a highly sensitive assay for plasma CPR that could detect as little as 0.03 ng/ml. After stimulation with glucagon, a significant increase in plasma CPR was observed in 10 of the patients whose beta-cell function had been evaluated as completely depleted by a conventional assay for plasma CPR. A clear inverse correlation was found between the secretory capacity of pancreatic beta-cells measured in this way and the degree of glycemic instability (r = -.74, P less than .01). Infusion of insulin at a rate of 0.15 U.kg-1.h-1 for 60 min caused a continuous decrease in the plasma glucose level, resulting in neuroglycopenia in 7 of the 10 CPR nonresponders but only 2 of the CPR responders. During insulin-induced hypoglycemia, plasma glucagon immunoreactivity did not increase in the CPR nonresponders but increased significantly in the CPR responders. A positive correlation was found between the minimal residual beta-cell capacity and the responsiveness of alpha-cells to hypoglycemia (r = .65, P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)