Hydrogen sulphide is known to be a potent inhibitor of the respiratory chain and leads to a depression of aerobic metabolism. Organisms living in sulphidic habitats such as shallow coastal water inlets have to cope with its toxic effects. Tn the Southern Baltic the species Marenzelleria viridis, native to brackish water habitats in North-East America has proliferated rapidly and become a characteristic faunal element. The polytrophic conditions cause a high concentration of sulphide in the sediment, where the animals inhabit J-shaped burrows. Since H2S is thought to depress aerobic metabolism, the metabolic rate of the animal was investigated by means of direct and indirect calorimetry. The heat production of adults was significantly enhanced when exposed to sulphide (250 mu mol l(-1)) and hypoxic conditions. In addition larvae of M. viridis increased their metabolic rate in the presence of sulphide if oxygen supply was sufficient. In the absence of oxygen the heat dissipation was only slightly raised reflecting different detoxification processes in the presence and absence of oxygen. The enhanced metabolic rate at hypoxic, sulfidic conditions may be due to detoxification processes which are coupled with an energy gaining system used by the animal itself or which may be associated with bacteria.