Changes in cytosolic proteins of the olfactory system (olfactory epithelium, olfactory nerve and olfactory bulb) and the telencephalon of masu salmon Oncorhynchus masou were analysed by two-dimensional polyacrylamide gel electrophoresis during parr-smolt transformation; parr, pre-smolt and full-smolt stages. In the olfactory system, several protein spots appeared and disappeared in the course of smolting. One protein spot in particular with an estimated molecular weight of 27 kDa and isoelectric point of 5.6 (M27) disappeared in common with the olfactory system during smolting. The disappearance of M27 was also observed in the telencephalon. These proteins, which appeared and disappeared, may reflect the changes in olfactory function during smolting. Simultaneously, the present study confirmed a salmonid olfactory specific protein of 24 kDa (N24), which existed in the olfactory system but not in the telencephalon, as a single neutral protein spot in masu salmon.