to play their ecological role in an ecosystem, a fully functioning olfactory system is vital for fishes. a fish's olfactory system is very sensitive to low concentrations of contaminants. a great deal of research has been conducted on the toxicity of metals on the olfactory systems of fish. while tightly-controlled laboratory studies are necessary to reveal the mechanism of toxicity, field-based studies using more realistic conditions are also needed to produce more environmentally relevant data for regulatory needs. this research has four main areas of focus: 1) to examine the effect of copper on gene transcription in olfactory tissues of yellow perch (perca flavescens); 2) to test if a modified diet can protect olfaction against copper toxicity in rainbow trout (oncorhynchus mykiss); 3) to examine chemosensory-mediated behaviours and gene transcription profiles in wild yellow perch from metal contaminated lakes; and 4) to investigate the potential recovery of yellow perch with contaminant-induced chemosensory function from metal contaminated lakes. we used gene transcription (a recently developed custom made micro-array for yellow perch and real-time pcr), neurophysiological testing (electro-olfactography (eog)), and behavioural assays (avoidance from alarm cues). in this research, yellow perch were exposed to elevated concentrations of copper for 3 and 24 hours. while 3 hours of exposure did not influence gene expression, 24-hour exposures to copper elicited a differential expression of 71 genes. of these 71 genes, differential expression of two subunits of na/k-atpase was further explored using real-time pcr in a time-series study. to investigate if increased dietary sodium can protect fish's olfaction against copperinduced olfactory impairment, rainbow trout were fed with diets having elevated concentrations of sodium ... the data produced by this thesis significantly improves our knowledge regarding the protection against metal induced olfactory toxicity as well as the recovery potential for impaired olfaction in fish. these results could be used to draft ecologically-relevant regulations that will protect fish inhabiting sensitive fresh water ecosystems.