The distribution of enkephalin-like immunoreactive (ELI) cell bodies and fibres in the brain of the teleost Salmo gairdneri L. was demonstrated with the indirect peroxidase-antiperoxidase immunocytochemical technique using a highly specific antiserum. In the telencephalon, ELI cell bodies were located in the area ventralis. In the diencephalon, they were found in the nucleus ventromedialis of the thalamus, nucleus lateralis tuberis, nucleus recessus lateralis, and nucleus recessus posterioris. In the mesencephalic tegmentum, ELI cell bodies were found in the nucleus of the rostral mesencephalic tegmentum, and in a group of neurons which was located dorsal to the nucleus of the rostral mesencephalic tegmentum. In the medial torus semicircularis, small numbers of immunoreactive cell bodies were found. In the cerebellum, numerous cell bodies were observed in the granule cell layer and at the border between the granular and molecular layer. ELI cell bodies were also seen in the nucleus tegmenti dorsalis lateralis and nucleus fasciculi solitarii. ELI fibres were widely distributed in the rainbow trout brain. The highest density of immunoreactive fibres was found in the area ventralis telencephali, the mesencephalic tegmentum, the stratum opticum of the optic tectum, the central gray of the brainstem, the caudal part of the fasciculi solitarii and the dorsal horn of the spinal cord. In the stratum fibrosum et griseum superficiale, stratum griseum centrale and stratum album centrale of the optic tectum, a moderate number of immunoreactive fibres was observed. In the olfactory bulb only a few immunoreactive fibres were present. No effect in the labelling was found after colchicine injections. These results provide the first complete mapping of the ELI in a fish brain. It is clear that enkephalins show a similar distribution pattern in Salmo gairdneri to that in other vertebrates; however, the number of ELI cell bodies in the fish brain is smaller than in land vertebrates. The distribution of enkephalins in specific hypothalamic nuclei, visual areas, and in the brainstem of the rainbow trout brain, suggests that these peptides are involved in the modulation of neuroendocrine and as well in visual and somatosensory functions.