The distribution of various opioid peptides derived from proenkephalin A and B was studied in the brain of the African lungfish Protopterus annectens by using a series of antibodies directed against mammalian opioid peptides. The results show that both Metenkephalin- and Leu-enkephalin-immunoreactive peptides are present in the lungfish brain. In contrast, enkephalin forms similar to Met-enkephalin-Arg-Phe, or Met-enkephalin-Arg-Gly-Leu, as well as mammalian alpha-neoendrophin, dynorphin A (1-8), dynorphin A (1-13), or dynorphin A (1-17) were not detected. In all major subdivisions of the brain, the overwhelming majority of Met-enkephalin- and Leu-enkephalin-immunoreactive cells were distinct. In particular, cell bodies reacting only with Leu-enkephalin antibodies were detected in the medial subpallium of the telencephalon, the griseum centrale, the reticular formation, the nucleus of the solitary tract, and the visceral sensory area of the rhombencephalon. Cell bodies reacting only with Met-enkephalin antibodies were found in the lateral subpallium of the telencephalon, the caudal hypothalamus, and the tegmentum of the mesencephalon. The preoptic periventricular nucleus of the hypothalamus exhibited a high density of Metenkephalin-immunoreactive neurons and only a few Leu-enkephalin-immunoreactive neurons. The distribution of Met-enkephalin- and Leu-enkephalin-immunoreactive cell bodies and fibers in the lungfish brain showed similarities to the distribution of proenkephalin A-derived peptides described previously in the brain of land vertebrates. The presence of Met-enkephalin- and Leu-enkephalin-like peptides in distinct regions, together with the absence of dynorphin-related peptides, suggests that, in the lungfish, Met-enkephalin and Leu-enkephalin may originate from distinct precursors.