The effects of sublethal copper on metabolism were in vestigated in bluegill (Lepomis macrochirus) by measuring whole body oxygen consumption in fish exposed to sublethal copper alone and in conjunction with a temperature increase. In vitro oxygen consumptions of liver, brain, and gill were also measured under these two conditions, as was the accumulation of copper in these tissues. In addition, the concentration of copper in bile was measured. Copper was found to decrease whole body oxygen consumption in animals exposed to copper alone, although the oxygen consumptions of tissues were not significantly altered. This indicates that copper is acting to decrease VO₂ at a higher level of integration than the individual tissues. In animals subjected to an increase in temperature as well as sublethal copper, oxygen consumption was higher than controls five days after the temperature was increased, indicating a delay in temperature acclimation. This increase was reflected in higher in vitro oxygen consumption in the liver and gill indicating that sublethal copper delays temperature acclimation by acting directly on the tissues. Tissue copper accumulation was seen first in the gills followed by accumulation in the liver. Copper was not found to accumulate in the brain. Increased copper levels were found in the bile at all tested exposure times. A discussion of the ecological implications of these findings is included. / M. S.