Biomarkers of metal exposure are well known, but how a suite of such biomarkers will respond if the metal is also an oxidizing agent or causes oxidative stress is unclear. This study compares the effects of copper and arsenic, two metals with different oxidizing potential, on freshwater invertebrates. Dreissena polymorpha and Asellus aquaticus were exposed to nominal concentrations of copper (100 microg/L) or arsenic (80 microg/L) over 7 days, and physiological stress was examined by measuring metallothionein (MT) induction, thiobarbituric acid reactive substances (TBARS), and Na(+)/K(+)-ATPase activity. Both species showed increased levels of MT during 7-day Cu exposure tests and transient changes in lipid peroxidation (TBARS) which decreased to control levels by day 7. Arsenic had no effect on TBARS and only a transitory effect on MT in D. polymorpha over 7 days, although it initially induced lipid peroxidation in A. aquaticus on day 3. No inhibition of the Na(+)/K(+)-ATPase enzyme was observed for exposed organisms, and baseline values reported here, for A. aquaticus, 1.1 micromol Pi/mg/h, and for D. polymorpha, 0.38 micromol Pi/mg/h, are probably the first reported for these species.