The distribution of mRNA encoding thyrotropin-releasing hormone (TRH) precursor in the brain of sockeye salmon was studied by in situ hybridization histochemistry using digoxigenin-labeled riboprobes as a basis to investigate its physiological functions in the salmon brain. Since seasonal variation in TRH gene expression was expected in relation to smelting or maturation, fish were sampled in February and October. In both groups, TRH mRNA was widely distributed in discrete brain regions including the internal cellular layer (ICL) of the olfactory bulb, postcommissural nucleus of area ventralis telencephali (Vp), nucleus preopticus parvocellularis anterioris (PPa), nucleus preopticus magnocellularis, dorsal zone of periventricular hypothalamus (Hd), torus semicircularis, and also the motor nucleus of vagus nerve in the medulla oblongata. TRH neurons in ICL and Hd are round and small with diameters of 5-10 μm. In contrast, TRH neurons in the ventral telencephalon and the preoptic area are medium-sized (10-20 μm), and appear to have multiple processes. Most of these cells are restrictively localized along the lateral margin of the preoptic nuclei. The number of TRH neurons in Vp and PPa was smaller in February than in October, suggesting a seasonal change of TRH neurons in the preoptic area. In the medulla oblongata, a cluster of large oval-shaped cells (20-30 μm) showed signals for TRH mRNA. The present results suggest that TRH may function as a neurotransmitter or neuromodulator involved in olfactory activity and also autonomic motor integration, in addition to neurohormonal control of secretion of pituitary hormones.