A system for computerized registration of heat production data and oxygen tension values of a differential 1 litre flow-through microcalorimeter (Setaram GF 108) is described. The linked values of oxygen concentration and heat production data were simultaneously stored on a computer. The aerobic and total heat production rates were thus measured of goldfish (Carassius auratus L.) during normoxia, hypoxia and anoxia exposure. Incidentally, a partial anaerobic response was observed in goldfish during normoxia. During severe hypoxia and anoxia, goldfish respond with a reduction of the metabolic rate and a strong increase of anaerobic heat production. From the linked oxygen consumption and heat production data, the oxycaloric equivalent was calculated. The described system enables us to study continuously the effects of environmental factors on the metabolic rate of aquatic organisms.