Met-enkephalin immunoreactivity was investigated with an indirect immunoperoxidase technique in the cervical spinal cord, brainstem and midbrain of the cat, paying special attention to pain-related nuclei. Different technical conditions were used to reveal preferentially met-enkephalin-containing fibres and terminals or perikarya. Immunoreactive fibres and terminals were revealed optimally in sections from control animals incubated with detergent (Triton X-100). Immunoreactive perikarya were revealed in colchicine treated animals. Comparison between different routes of administration showed that local injections of colchicine are needed to reveal optimally immunoreactive perikarya in nuclei located far from the ventricles. Met-enkephalin-containing fibres and terminals are widely distributed in the posterior brain and spinal cord. The densest network of immunoreactive fibers are observed in the superficial layers of the cervical spinal cord and the caudal trigeminal nucleus, in the nucleus of the solitary tract, the nucleus of the facial nerve, the nucleus of the prepositus hypoglossi, the nucleus raphe pallidus, the medial vestibular nucleus, the interpedoncular nucleus and the substantia nigra. A moderate staining of fibres is observed in various nuclei including the ventral horn of the spinal cord and caudal trigeminal nucleus, the brainstem and midbrain reticular formation, the inferior olivary complex, the nucleus of the descending trigeminal tract and the periaqueductal grey. Met-enkephalin-containing perikarya are present in all the nuclei cited before, except in the inferior olivary complex. The densest aggregation of enkephalin-like perikarya is observed in the nucleus raphe magnus, nucleus raphe obscurus, nucleus raphe pallidus, nucleus reticularis gigantocellularis pars alpha and nucleus reticularis lateralis. The general distribution of enkephalin-containing structures in the cervical spinal cord, brainstem and midbrain of the cat appears very similar to that of the rat except in the substantia nigra where met-enkephalin cell bodies are found in the cat but not in the rat. In particular the pain-related nuclei present a similar distribution of the peptide in the two species; however, met-enkephalin-containing cell bodies are much more numerous in the cat than in the rat (notably in the reticular formation). Similar types of met-enkephalin innervation occur in the dorsal and intermediate grey of the spinal cord and of the caudal trigeminal nucleus supporting further that the functional organizations of these regions are closely related.