The secretion of plasma C-peptide after intravenous glucagon stimulation was studied in 15 insulin-treated diabetic patients with onset of diabetes after the age of 30. The mean stimulation of C-peptide secretion caused by glucagon given in the fasting state and by a standardized breakfast were similar. Low blood glucose values (less than 3.5 mmol/l) were found to suppress the stimulating action of glucagon on the pancreas almost completely. When the glucagon test was performed 1.5 hours after a standardized breakfast, the mean concentration of plasma C-peptide was 62% higher than in the test in the fasting state, showing that the stimulating actions of glucagon and breakfast on the secretion of insulin are additive. The results indicate that when determining the level of plasma C-peptide after stimulation with glucagon, in order to distinguish between insulin-dependent and non-insulin-dependent diabetic patients, it is critical to take into account the consequence of low blood glucose values and to standardize the test conditions in regard to pre-test meals.