In obese rodents increased daily food intake leading to accumulation of adipose tissue is frequently accompanied by increased meal size and loss of the normal diurnal variations in feeding pattern. Increased meal size of obese rats may be due to decreased sensitivity to factors which elicit satiety. We compared Zucker obese and lean rat feeding behavior responses to octapeptide of cholecystokinin (OP-CCK), a peptide shown to decrease meal size in several species. Obese rats were less sensitive than lean rats to OP-CCK (.06, .25 and 1.0 μg/kg/meal) injected before each of four consecutive scheduled meals in the light portion of the diurnal cycle, when obese meal size was larger than lean. However, neither obese nor lean rats responded to injection of the same doses of OP-CCK during meals in the dark, when average meal size was larger than during the light and when average meal size of the obese rats was similar to that of lean rats. In both obese and lean rats injection of OP-CCK affected daily feeding pattern. Obese and lean Zucker rats are less sensitive to OP-CCK when meal size is larger, whether this is due to phase of the diurnal cycle (dark vs. light in both obese and lean rats) or phenotype (obese vs. lean rats in the light).