Since the discovery of the satiety effect of CCK in 1973, progress has been made and problems have been encountered. The progress has included the accumulation of strong, indirect evidence that exogenous CCK acts in the abdomen to activate vagal afferent fibers and that exogenous CCK may be useful in the treatment of bulimia and obesity in humans. The most pressing problem is the current lack of evidence for the hypothesis that the satiety effect of exogenous CCK reveals a physiological function of endogenous CCK released by food entering the small intestine during a meal. Since clarification of this problem and exploitation of the current progress seem possible with current ideas and techniques, the satiety effect of CCK should continue to receive considerable experimental attention.