To evaluate the impact of glucagon deficiency on the response to glucagon replacement, we infused physiological doses of glucagon (1.25 ng/kg X min) into 9 totally pancreatectomized (PX) diabetic patients (C-peptide, undetectable) 1) for 24 h during their usual diet and insulin regimen and/or 2) for 6 h in a fasted insulin-withdrawn state. During both glucagon infusions, plasma glucagon rose from 46 +/- 2 (+/- SE) pg/ml (0-10% 3500 mol wt glucagon) to 112 +/- 9 pg/ml. In the 24-h study (n = 4), glucagon significantly increased mean 24-h glucose levels (272 +/- 27 mg/dl; P less than 0.05) and glycosuria (29 +/- 5 g/day; P less than 0.01) compared to preinfusion (158 +/- 14 mg/dl and 4 +/- 4 g/day, respectively) and postinfusion (200 +/- 35 mg/dl and 3 +/- 2 g/day) control periods. Blood ketones did not change. The 24-h glucagon infusion significantly lowered the fasting levels of the glucogenic amino acids aspartate (43%; P less than 0.01), threonine (46%; P less than 0.05), serine (46%; P less than 0.02), glycine (47%; P less than 0.01), and methionine (34%; P less than 0.02). Fasting alanine levels decreased from 835 +/- 236 to 393 +/- 66 microM (P less than 0.05). The 6-h glucagon infusion caused a 101 +/- 14 mg/dl maximal plasma glucose increment in PX (n = 8) vs. 33 +/- 11 in 5 insulin-withdrawn type I diabetic patients serving as controls (P = 0.022). Furthermore, when glucagon was infused at a higher rate (3 ng/kg X min) in 12 additional type I diabetic patients, the mean maximal plasma glucose increment (54 +/- 15 mg/dl) was still less than half that in PX, despite a 3-fold higher infusion plasma glucagon level (326 +/- 37 pg/ml). The 6-h glucagon infusion caused a significant decrease in the concentrations of glucogenic amino acids in the glucagon-deficient patients, but not in the type I diabetic patients. We conclude that 1) glucagon replacement in the PX patient markedly alters blood glucose and glucogenic amino acids, but not ketone levels; and 2) the metabolic response to glucagon is considerably more pronounced in PX patients than in type I diabetic patients. These data suggest that glucagon responsiveness is enhanced in the chronic hormone-deficient state.