In a group of 4 dogs the infusion of 10 U of secretin per minute for a 20-minute period during alanine-induced hyperglucagonemia (1 mmole of alanine per kg over 60 minutes) was associated with a prompt and uniform suppression of both peripheral and pancreatico-duodenal vein glucagon concentrations to or below basal levels, and a rapid return to stimulated levels when secretin was terminated. The secretininduced fall in glucagon was associated with a decline in plasma glucose which averaged 15 mg/100 ml, and which could not be explained by increased insulin secretion. 10 U of secretin per minute infused simultaneously with approximately equimolar amounts of pancreozymin (7.5 Ivy units per minute) failed to reduce the pancreozymin-stimulated increase in glucagon, however. 1 Supported by NIH Grant AM 02700-14; Bristol Myers Company, New York, NY; Dallas Diabetes Association, Dallas, Texas; Eli Lilly Company, Indianapolis, Indiana; Hoechst Pharmaceutical Company, Somerville, N.J.; Mead Johnson Research Center, Evansville, Indiana, Pfizer Laboratories, New York, N.Y.; The Upjohn Company, Kalamazoo, Michigan; and Wm S. Merrell and Company, Cincinnati, Ohio.