We investigated the gene expression dynamics of several hypothalamic neuropeptides associated with appetite regulation when rats are anticipating being fed a corn-oil emulsion. For 5 days at the same hour each day, rats were fed 5% corn oil emulsifi ed with 0.3% xanthan gum or the vehicle for 20 min. On Day 6, the 5% corn oil emulsion or the vehicle (Vehicle) was presented to the rats, some of which (Oil-intake) were allowed to eat it and some of which (Oil-anticipation) were kept from eating it. Despite waiting a corn-oil, the mRNA levels of proopiomelanocortin (POMC), a β-endorphin precursor, and orexin showed increases, and high levels of mRNAs of POMC and orexin were maintained for 30 min after the corn-oil was placed before the rats, and only gradually decreased through 150 min. However, the mRNA levels of POMC and orexin in the hypothalamus were decreased within 30 min after starting to ingest the corn-oil emulsion. These results suggest that POMC and orexin mRNA expression was induced by the anticipation in rats after learning the palatability of 5% corn oil emulsion, and the induced mRNA expression based on the anticipation was maintained for at least for 30 min as the rats eagerly waited for ingestion.Obesity resulting from excessive energy ingestion is a serious health issue. Despite warnings about excessive ingestion of high-fat foods, there currently appears to be no decrease in consumption of such foods (3,11,18,22). It is thought that many mammals prefer to consume high-fat foods due to their pleasant taste, flavor, and texture. In short-term 2-bottle choice tests, mice prefer vegetable oils to sucrose, itself a highly palatable substance (20). In addition, conditioned place preference (CPP) tests have demonstrated a reinforcing effect of corn oil on mice (6). In comparison to rats fed on regular chow diets, rats fed on a high-fat diet show a greater food intake and became overweight as a result (1). Understanding why mammals, including humans, prefer high-fat foods will provide important information for regulating oil ingestion and thus maintaining good health.Several neuropeptides produced in the hypothalamus mediate feeding behavior. These include proopiomelanocortin (POMC), produced in arcuate nucleus of hypothalamus, and physiologically co-active peptides like beta-endorphin, adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH) (4, 17). We discovered that betaendorphin, an opioid peptide, is released just after dietary oil ingestion in rats (12). Moreover, the gene expression of POMC (a precursor of beta-endorphin) increased just before dietary oil ingestion (12).