This study was undertaken to determine whether casein, compared with its constituent amino acids, given at the onset of a meal, would influence intake due to cholecystokinin (CCK) or opioid activity. Male Sprague-Dawley rats (n = 80; 225 g) were given either premeal loads of casein or its constituent amino acids and treated with opioid or CCK antagonists in a 2 x 4 factorially designed experiment. During a 21-d period, rats were meal-fed by restricting access to food to 5 h/d. The rats were fed the AIN-93 diet with soy isolate substituted for casein as the dietary protein source. On d 7-21, rats were given oral premeal loads of 5 mL of a 50 g/L casein or constituent amino acid solution before meal-feeding. On d 14-21, 20 rats were injected intraperitoneally with one of the following treatments: saline, naltrexone (l mg/kg), naloxone methiodide (5 mg/kg) or lorglumide (1 mg/kg) before the premeal load and feeding. Antagonist treatments increased intake (P < 0.05) by 15.3% compared with saline treatment (7.82 vs. 9.02 g/d) in rats given premeal loads of casein. Intake of rats given premeal loads of amino acids was not influenced by antagonists. At 2 h after feeding on d 21, the rats were killed, bled and eviscerated. Effects of antagonists on stomach and intestinal mass, digesta contents and fecal output were also dependent on the type of premeal load, indicating that gastric retention of digesta due to casein was mediated by CCK and opioids. Body weight accretion, liver, and epididymal fat mass and blood concentrations of specific amino acids changed in the same manner as intake (P < 0.05). Serum insulin was greater (P < 0.05) in casein-treated rats and reduced (P < 0.01) by opioid antagonists. Satiety associated with premeal loads of casein is related to changes in gastrointestinal function of meal-fed animals and involves both opioid and CCK regulation.