Using a specific glycine antiserum, we determined the localization of glycinergic neurones and fibres in the cat brain stem reticular formation. We visualized a large number of glycine-immunoreactive cell bodies and fibres in the medullary reticularis gigantocellularis, magnocellularis, paragigantocellularis lateralis and parvocellularis nuclei. The pontis oralis and caudalis and the raphe magnus nuclei also contained a large number of glycine-immunoreactive fibres but fewer neurones. Using a double staining method, we further observed glycine-immunoreactive boutons over (1) noradrenergic neurones in the locus coeruleus complex and the ventrolateral and dorsomedial medulla, (2) serotoninergic neurones in and outside the raphe nuclei and (3) cholinergic neurones in the pedunculopontine and laterodorsal tegmental nuclei. These results suggest that glycinergic neurones in the reticular formation may be involved in aspects of paradoxical sleep, including the general muscle atonia seen during this sleep state.