The effect of hydrogen peroxide on the rate of tissue oxygen consumption, on intracellular pH (pHi), and on malondialdehyde (MDA) accumulation was studied in isolated body wall tissue of the lugworm Arenicola marina (L.). H2O2 effects were investigated at various levels of pHi by changing medium pH (pHe).The largest decrease of tissue oxygen consumption (by 17% below controls), as well as the highest degree of MDA accumulation (4-fold compared to control values) after H2O2 exposure were found at acidic pHe of 6.4. This was attributed to the higher redox potential of H2O2 in acidic solutions. Oxygen consumption at alkaline pHe (8.5) was not affected by H2O2. MDA accumulation in the tissue was considerably lower than at pHe 7.4 or 6.4. Despite pH dependent alterations of H2O2 redox potential, we observed more or less constant pHe independent acidification of the tissue upon exposure to H2O2. We attributed the acidification to an inhibition of ATP consuming proton equivalent ion transport across the cellular membrane. Inactivation of carrier proteins is discussed to be responsible for the decrease in tissue oxygen consumption. However, with a larger effect on oxygen consumption at acidic pHe values the latter may not be the only explanation, but additional impairment of other energy demanding processes may be involved.