<p>Teleosts take up metals by two major pathways: gills and/or gut. Past research is heavily focused on branchial uptake despite evidence that the gastro-intestinal tract (GIT) is the dominant route in some natural environments. To address this information gap, my thesis characterizes uptake mechanisms of Cd and Ca along the GIT of <em>Oncorhynchus mykiss</em>. Toxic effects of Cd, protective effects of Ca against Cd uptake, and Cd distribution within fish after ingestion are also explored.</p> <p>Four-week dietary Cd exposure affected growth and Ca regulation, while causing toxicity at the subcellular level. Elevated Ca in diets protected against accumulation and altered subcellular handling of Cd. Pre-exposure to different diets changed unidirectional uptake and binding rates of Cd and Ca, although they remained highly correlated. Binding rates of Cd to mucus can predict absorption rates of Cd<strong>.</strong></p> <p>A variety of <em>in vitro </em>and <em>in vivo </em>experiments on four distinct GIT segments (stomach, anterior-, mid-, and posterior- intestine) were undertaken to determine specific mechanisms of Cd and Ca uptake in freshwater trout. Cd transport was unaffected by solvent drag, but was stretch- and temperature- sensitive. Strong evidence for a common pathway for Ca and Cd transport was obtained. Cd also appeared to be taken up in part by zinc (ZIP-like) transporters and the divalent metal transporter DMT1. Ca uptake along the GIT appeared to be carrier-mediated, time- and concentration- dependent, but was not affected by solvent drag, or by Na concentration. Mucosal Cd did not inhibit Ca uptake. Ca, but not Cd, uptake rates were much lower in sea water-acclimated trout. Seawater and freshwater fish accumulated similar whole body Cd concentrations when fed contaminated diets for three weeks, but the majority of Cd in seawater trout remained in the posterior intestine tissue, while freshwater trout had higher internal burdens.</p> <p>This thesis advances the knowledge of metal uptake along the GIT of trout.</p> / Doctor of Science (PhD)