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Abstract . <br />Effects of two potential environmental contaminants, copper <br />and zinc, on olfaction of Colorado squawfish (Ptychocheilus <br />lucius) were investigated by exposing fish to sublethal <br />concentrations of each toxicant for 24 or 96 h, and evaluating <br />olfactory ability by challenging fish with a fright-pheromone <br />extract. Olfactory inhibition was inferred based on presence or <br />absence of a fright reaction. Olfactory inhibition increased as <br />a function of toxicant concentration after 24-h exposure to <br />copper and zinc, and 96-h exposure to copper, but not after 96-h <br />exposure to zinc. Concentrations of copper and zinc estimated to <br />inhibit olfaction in 1% of test organisms were 0.888 (0.155, <br />5.12) µg/L copper and 135 (16.1, 1150) µg/L zinc for 24-h <br />exposures, and 15 (5.69, 43.6) µg/L copper for 96-h exposure. <br />Results suggest that sensitivity to toxicant exposure declined as <br />duration of exposure increased. Protective mechanisms induced by <br />toxicant exposure may have been responsible for reduced <br />sensitivity of olfactory receptors in 96-h exposures. Systematic <br />surveys of olfactory receptor cells confirmed results of <br />behavioral assays. The USEPA water quality criteria may not <br />protect olfactory ability of wild Colorado squawfish. <br />ii <br />