In the chicken, glomus cells are widely distributed in the carotid body and in the wall of the common carotid artery and around its branches. The cells located in the wall of the common carotid artery express intense immunoreactivity for neuropeptide Y (NPY). They contain abundant dense-cored vesicles ranging from 70 to 220 nm in diameter. In this study, we examined ultrastructural localization of NPY in the glomus cells by using the postembedding immunogold method. Gold particles representing immunoreactivity for NPY were specifically localized on the dense-cored vesicles of the glomus cells. In addition, the localization of NPY mRNA in the glomus cells was examined by in situ hybridization with digoxigenin-labeled chicken NPY cRNA probe. A strong hybridization signal for NPY mRNA was detected in the glomus cells located in the wall of the common carotid artery. Few glomus cells of the carotid body, however, displayed labeling for NPY mRNA. Northern blot analysis with the chicken NPY exon 4 probe demonstrated that a single band for NPY mRNA was present in the poly (A)Ο©RNA isolated from the common carotid artery where the glomus cells were distributed. Furthermore, the expression of NPY mRNA in the common carotid artery was confirmed by the reverse transcription-polymerase chain reaction. These results indicate that the chicken glomus cells are able to produce NPY but that the level of its translation varies according to the location of the cells.