Pancreatic glucagon and cholecystokinin octapeptide (CCK-8) were intravenously infused (1 ml/min for 10 min) alone or in combination beginning 15 min after normal-weight men had eaten a 500-ml tomato soup preload and 5 min before they were served a lunch of macaroni and beef with tomato sauce. Infusion of approximately 3 ng.kg-1.min-1 glucagon or approximately 2 ng.kg-1.min-1 CCK-8 each reduced test meal size. However, simultaneous infusion of these peptide doses reduced meal size less than the sum of the peptides' individual effects. Infusions of approximately 1.5 ng.kg-1.min-1 glucagon or approximately 1 ng.kg-1.min-1 CCK-8 had neither individual nor interactive effects on meal size. Psychophysical ratings failed to detect nonspecific side effects after any of the infusions. That exogenous glucagon and CCK-8 each reduced meal size without side effects suggests that these peptides may participate in the physiological control of human appetite; that their simultaneous infusion resulted in an infra-additive reduction in meal size suggests that they can interact antagonistically.