The aim of this study was to assess the acute, sublethal effects of chemicals on fish from freshwater habitats. To this end goldfish (Carassius auratus) were exposed for up to 48 h to 2,4-dinitrophenol at concentrations well below the LC(50) (96 h). In some experiments, fish were exposed to 2,4-dinitrophenol plus hypoxia/anoxia as an additional metabolic stress in order to enhance the effects of the chemical. Microcalorimetry proved very sensitive for detecting metabolic changes induced by 2,4-dinitrophenol. At 6 mg . l(-1) (corresponding to 1/4 LC(50)) the chemical had no significant effects on behaviour, motor activity and ventilation frequency, whereas the heat flow rate was markedly increased. Zebrafish Brachydanio rerio were unable to tolerate severe hypoxia, whereas goldfish proved very tolerant of hypoxia/anoxia. In goldfish, hypoxia induced a marked decrease in heat production, to less than 30% of the normoxic rate. Postanoxic recovery after 3-7 h of anoxia was rapid and complete; it was characterised by a transient period of excess heat. 2,4-Dinitrophenol affected the rate of aerobic heat production. It had no significant effect during hypoxia/anoxia but it markedly increased the heat flow rate during postanoxic recovery. As an adaptation to anoxia, goldfish produce and excrete ethanol during anaerobiosis. This process was not affected by 2,4-dinitrophenol. Microcalorimetry appears to be a sensitive method to detect metabolic effects of environmental chemicals.