The present study was conducted to identify the mechanism about the regulation of appetite by examining the expression patterns of neuropeptide Y in the hypothalamus of mice either fasting mouse for 24 hours or with anorexia mutant mouse. In order to investigate the patterns of expression of neurpeptide Y, immunohistochemistry was employed for measurements at the tissue level, along with the molecular biological techniques of reverse transcription polymerase chain reaction(RT-PCR) and dot blotting. The results of this study are as follows. The level of expression of neruopeptide Y, a neuropeptide known to enhance appetite, was shown to be lowered in the arcuate nucleus(ARC), paraventricular nucleus(PVN), lateral hypothalamic area(LHA), and dorsomedial hypothalamic nucleus(DMN) in both the fasting and anorexia mutant groups when measured via immunohistochemistry, a tissue-level method. RT-PCR and dot blotting, the molecular biological methods employed in this study, revealed that the level of neuropeptide Y mRNA in the entire hypothalamus was similar in the control and fasting groups and lower in the anorexia mutant group. The results of the present study showed that while the levels of expression of the neuropeptide Y in the various hypothalamic regions studied did not exhibit regular increases or decreases when measured immunohistochemically. But the entire hypothalamus via molecular biological methods showed that the changes in these levels were more definite in the anorexia mutant group than in the fasting group.