Prior data have demonstrated differential roles for cholecystokinin (CCK) 1 receptors in maintaining energy balance in rats and mice. CCK1 receptor deficiency results in hyperphagia and obesity of Otsuka Long-Evans Tokushima Fatty (OLETF) rats, but not in mice. To ascertain the role of CCK1 receptors in high fat diet (HFD)-induced obesity, we compared alterations in food intake, body weight, fat mass, plasma glucose and leptin levels, and patterns of hypothalamic gene expression in OLETF rats and mice lacking CCK1 receptors in response to a 10-week exposure to HFD. As compared with Long-Evans Tokushima Otsuka (LETO) control rats, OLETF rats on HFD had sustained overconsumption over the 10-week period. High fat feeding resulted in greater increases in body weight and plasma leptin levels in OLETF than in LETO rats. In situ hybridization determinations revealed that while HFD reduced neuropeptide Y (NPY) mRNA expression in both the arcuate nucleus (Arc) and the DMH of LETO rats, HFD resulted in decreased NPY expression in the Arc but not in the DMH of OLETF rats. In contrast to these results in OLETF rats, HFD increased food intake and induced obesity to an equal degree in both wild type and CCK1 receptor^-/- mice. NPY gene expression was decreased in the Arc in response to HFD, but was not detectable in the DMH in both wild type and CCK1 receptor^-/- mice. Together, these data provide further evidence for differential roles of CCK1 receptors in the controls of food intake and body weight in rats and mice.