Using a highly specific antiserum to Glycine and a very sensitive immunohistochemical technique with streptavidin-HRP, we visualized for the first time a considerable number of glycine immunoreactive cell bodies and fibers in the cat medulla oblongata. These results suggest that glycine may play an essential role in nearly all the physiological functions involving the medulla oblongata, including the muscular atonia occurring during paradoxical sleep.