The outcome of 97 paired glucagon and meal tests was related to the prevailing fasting blood glucose concentrations on the two test days. At blood glucose concentration less than 7 mmol/l both the C-peptide responses and the maximal (6 min) C-peptide concentrations during the glucagon tests were significantly lower than the corresponding values found during the meal tests. During the glucagon tests, a direct relationship was found between the responsiveness of the pancreatic beta-cells and fasting blood glucose values when these were between 3 and 7 mmol/l. No significant difference was found between the outcome of the two tests when the fasting blood glucose concentration was greater than 7 mmol/l. The results indicate that the outcome of the glucagon test in Type I patients depends upon the prevailing fasting blood glucose concentration and that the predictive value of the glucagon test as to how the beta-cells will respond to a meal during everyday life is low when fasting blood glucose is less than 7 mmol/l.