Juvenile common carp were exposed for 28 days to three different sublethal copper concentrations (0.20 microM, 0.55 microM, and 0.80 microM). Food consumption was monitored on a daily basis during the exposure period, while growth, copper accumulation, energy stores, and nucleic acid contents were assessed weekly. Copper exposure to 0. 80 microM affected both growth and feeding behavior in common carp. At 0.55 microM, growth was affected despite normal food consumption. Even at the lowest copper concentration (0.20 microM), metabolic demand for the fish increased, challenging the carp with an increased demand for food. Copper accumulation mainly occurred in the liver, reaching an equilibrium between uptake and excretion after 1 month of exposure. Substantial biochemical changes were observed at the two highest copper exposure concentrations, but the correlation between growth rate and RNA:DNA ratio was poor considering the substantial differences in growth rate. The use of the RNA:DNA ratio as a sensitive biomarker is questioned.