The neurophysiological responses to 2-ml intragastric saline loads and 100-pmol celiac artery infusions of cholecystokinin (CCK) were obtained from 20 vagal afferent fibers in 14 rats. Two groups of fibers were identified. Discharge rates of group I fibers (n = 16) were significantly increased by gastric loading, adapted slowly to maintained gastric volume, and were inhibited by load withdrawal. CCK elicited a significant increase in the discharge rate of these group I fibers. Prior exposure to CCK nearly doubled the response of these fibers to a subsequent gastric load. In contrast, group II fibers (n = 4) increased firing rate only during infusion of a gastric load and showed rapid adaptation and no response to CCK. CCK failed to alter subsequent responses to gastric loads in these fibers. These results 1) demonstrate an integration of signals elicited by exogenous CCK and gastric loads at the level of vagal afferent fibers and 2) imply that aspects of CCK's inhibition of food intake may derive from CCK's ability to mimic and amplify vagal afferent activity provoked by meal-related gastric events.