Effects of total pancreatectomy on plasma glucagon, insulin and glucose responses to arginine were determined in 5 dogs. Portal vein and femoral artery samples were obtained in response to an arginine infusion (10 g/30 min) prior to, 1 h, 1 day and 1 week after pancreatectomy. Glucagon was measured using pancreatic-specific antiserum 30K (Unger, Dallas). Before pancreatectomy arginine significantly increased portal vein glucagon from 373 ± 36 to 595 ± 31 pg/ml and femoral artery levels from 233 ± 28 to 342 ± 74 pg/ml. Portal vein and femoral artery insulin concentrations of 74 ± 21 and 17 ± 3 µU/ml increased significantly to 173 ± 64 and 31 ± 7 µU/ml. Glucose levels did not change. One h after pancreatectomy, portal vein glucagon decreased to 121 ± 15 pg/ml but increased to 230 ± 42 pg/ml after arginine. Elevated blood glucose and the necessity for insulin treatment established the adequacy of pancreatectomy. Furthermore portal vein insulin levels were undetectable and unresponsive to arginine or a combination of glucose, glucagon, and tolbutamide 1 week after pancreatectomy. One day after pancreatectomy arginine significantly increased portal vein glucagon from 343 ± 42 to 776 ± 152 pg/ml. One week after pancreatectomy basal glucagon values were 374 ± 30 in the portal vein and 360 ± 49 in the femoral artery and responded to 1226 ± 641 and 825 ± 270 pg/ml, respectively, with arginine. Chromatography of plasma from one pancreatectomized dog on Sephadex G-50 after arginine stimulation revealed that much of the material crossreacting with antibody 30K was eluted from the column earlier than either ¹²⁵I-insulin or ¹²⁵Iglucagon. In contrast, peak glucagon activity in plasma obtained from a normal human given arginine eluted from the column between the peak of ¹²⁵I-insulin and ¹²⁵I-glucagon; glucagon added to human plasma also was recovered in this same area between the ¹²⁵I-insulin and ¹²⁵I-glucagon peaks. These results suggest that some of the material that reacted with 30K antibody and which increased after pancreatectomy in response to arginine has a molecular weight greater than pancreatic glucagon. At autopsy no pancreatic tissue could be identified. Thus, after pancreatectomy, validated by absent insulin responses, the glucagon response to arginine was normal or increased. Since arginine is not thought to increase intestinal glucagon-like immunoreactive material, the source and nature of the material measured as glucagon after pancreatectomy is unknown, but may be important to any understanding of plasma glucagon measurements. (Endocrinology96: 678, 1975)