Food and water intake of free-feeding rats with indwelling intraperitoneal catheters connected to infusion pumps was continuously monitored and recorded by a microcomputer-based data acquisition system. Initially, at the start of every spontaneous meal for 4 days, each rat was infused with 0.27 ml of physiological saline. Saline infusion did not affect any feeding or drinking patterns, and the rate of weight gain remained unchanged. For 6 subsequent days, the octapeptide of cholecystokinin (CCK-8, 1.1 micrograms/meal) dissolved in physiological saline was infused at the onset of each meal. CCK-8 infusion caused a dramatic shift of patterns of food intake. Average meal size was reduced by at least 44%, whereas daily meal number increased by 162% or more for all 6 days of CCK-8 infusion. Total daily food intake recovered to predrug levels by the 4th day of CCK-8 infusion, primarily due to increased feeding frequency. Average body weight dropped by 12.4 g on the 1st day of CCK infusion, but over the following 5 days the growth rate was not different from the base-line predrug rate. With discontinuation of CCK-8 infusion all meal patterns returned rapidly to normal and body weight immediately recovered.