Activity levels of 2 antioxidant enzymes, catalase (CAT) and superoxide dismutase (SOD), and the rate of oxygen consumption were investigated in body wall tissue of the capitellid polychaete Heteromastus filiformis in response to the variability of abiotic factors in the worm's intertidal habitat. A head-down deposit feeder, H. filiformis displayed oxyconformity between 1.3 and 13 kPa (10 to 100 torr) P(O2) under laboratory conditions. An extremely low standard metabolic rate (SMR) equivalent to an oxygen consumption of 0.1 μmol O2 g-1 fw h-1 (fw = fresh weight) is consistent with the capacity of the worms to colonize anoxic and potentially sulphidic sedimentary environments. Maximal M(O2) was 0.3 ± 0.09 μmol g-1 fw h-1 for small (< 300 mg body fw) and 0.25 ± 0.09 μmol g-1 fw h-1 for large (> 500 nag body fw) worms. CAT and SOD activities were higher in summer than in winter worms. Under laboratory conditions, SOD activity in winter worms was not inducible upon acclimation to elevated temperatures, while CAT activity was significantly higher at 20 than at 5°C. Summer worms were unaffected by temperature reduction with respect to CAT activities, while SOD activity was significantly reduced upon cooling. Under laboratory conditions, hypoxia as well as elevated P(O2) led to a significant increase in CAT activity, while changes in SOD activity were marginal. Experimental hydrogen peroxide (H2O2) exposure resulted in an increase in CAT activity, whereas hydrogen sulphide (H2S) led to a decrease in CAT activity only if applied under anoxia. SOD activities of H. filiformis were insensitive to H2S in vivo. In situ exposure to elevated H2O2 concentrations confirmed that just 1 tidal emersion period was sufficient to cause the inducing effect of reactive oxygen species on CAT activities in the worms' natural habitat. It is concluded that short-term changes in CAT activity are triggered by specific environmental stress factors, like fluctuating P(O2) and hydrogen peroxide concentrations in the upper sediment layer. In contrast, SOD did not respond spontaneously under experimental conditions, but under in situ conditions at the sediment surface of an intertidal sandflat a shift of SOD activity occurred towards the end of an ebb tide emersion period, which led to an increase of SOD activity in the tail compared to the head end of the worms. It is hypothesized that short-term variations in P(O2) and temperature at the sediment surface, in combination with the vertical gradients of P(O2), H2S, temperature, and pH, elicit the observed changes in SOD activity.