UV-dependent hydrogen peroxide formation can lead to high levels of hydrogen peroxide in tidepool waters. The effect of ambient hydrogen peroxide on the rate of oxygen consumption and on the levels of intracellular pH, lactate and high energy phosphates was studied in the shrimp, Crangon crangon. Exposure to elevated hydrogen peroxide levels caused a decrease in metabolic rate by an average value of 26% in intact animals and by up to 60% in isolated preparations of the tail muscle. Muscle intracellular pH decreased by 0.1-0.2 pH units in both isolated tissue and intact animal. The maintenance of lactate and ATP at control levels strongly suggests that the acidosis was not associated with anaerobic proton production and, as a corollary, must be explained through an effect of hydrogen peroxide on proton equivalent ion exchanges mechanisms. Future studies will attempt to elucidate the nature of the effect and whether the associated acidosis leads to the observed decrease in aerobic metabolic rate.