The ability of Atlantic salmon to return to their natal stream to spawn after a long distance marine feeding migration represents a tremendous feat of navigation and has important consequences for the maintenance of population structure in the species. While the orientation mechanisms used in the open ocean are poorly understood, olfaction is known to be key to accurate homing in the coastal and riverine stages of the return migration. Juvenile salmon appear to imprint upon the odours of the homestream, including those of other fish in the population, particularly during the period immediately before commencing the migration to sea [known as the parr-smolt transformation (PST)]. Here we propose to test a model of the role of odorant receptor (OR) gene expression in generating homing specificity in Atlantic salmon. Preliminary data suggests that there are changes in OR gene expression during PST. We will confirm these initial results for a larger number of OR genes, investigate changes in OR gene expression during other periods that are potentially sensitive for olfactory imprinting and test the effect of exposure to different population-specific odours, administration of exogenous thyroid hormone and exposure to high water flow levels on OR gene expression.