CCK-33 was infused intravenously to groups of 9 lean and 9 obese volunteers in doses that elicited plasma CCK concentrations in the physiological range. The effect of these infusions on food intake and satiety signals was compared with the effect of saline infusions in the same subjects. Food intake (486 +/- 52 g; mean +/- S.E.M.) was slightly, but not significantly decreased (553 +/- 55 g after saline), and hunger and fullness feelings after eating were unaffected, in both of the two groups. We conclude that the infusion of CCK-33 to plasma levels comparable to those observed after a fatty meal does not have a major effect on food intake and postprandial hunger feelings.