The immunohistochemical distribution of thyrotropin-releasing hormone-like immunoreactivity (TRH-ir) in the brain and pituitary of the sea bass (Dicentrarchus labrax) was examined on cryostat sections of tissues perfuse fixed in a formaldehyde-glutaraldehyde mixture. TRH-ir fibres were found in many areas of the brain: dorsal and ventral telencephalon, preoptic and tuberal hypothalamus, thalamus, midbrain tegmentum, optic tectum, and medulla oblongata. In the hypothalamus the densest area of innervation was the nucleus anterioris tuberis and medial nucleus recessus lateralis, where small TRH-ir cell bodies were also found. In the pituitary gland, TRH-ir fibres were numerous in the posterior neurohypophysis, and these appeared to form varicosities between groups of melanocorticotropic cells of the pars intermedia. No clear relationship was seen between TRH-ir fibres and the thyrotropic cells, or any other cell type of the pars distalis. In the brain stem a notable feature was the prominent innervation of groups of motoneurons by beaded TRH-ir fibres. These observations suggest that in teleost fishes the role of TRH may be related to pars intermedia function, rather than the thyrotropin- or prolactin-releasing function established in tetrapods. In addition the tripeptide may act as a central neurotransmitter involved in sensory and autonomic motor integration.