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Oxygen consumption of carp acclimated at 10 and 20" C has been measured under routine conditions. Some compljcations and precautions necessary in continuous flow respirometry are discussed. Routine VO,, at different levels of hypoxia have been determined. Individual variation leads to scatter in the data and different methods of plotting the relationship between Vo2 and Po, .are proposed; attention is drawn to differences between inlet (or ambient) Po, and inspired Po . Using certain criteria a ' critical ' oxygen tension of about 95 mm Hg was found at 20"e; Qlo values are about 2 at normoxia and some suggestions of an increase near to the critical oxygen tension were found. Blood samples from the dorsal aorta showed rising Pa of 16 mm Hg which increased to 70-80 mm Hg when Pinsp was 90 and they then fall as*the inspired oxygen is lowered. During periods of deep hypoxia (25 mm Hg) blood lactate concentration increases steadily and indicates an increasing dependence on anaerobic mechanisms. I. INTRODUCHONCarp, Cyprinus carpio, are well known to be fish which can survive a wide range of environmental temperatures and levels of ambient oxygen concentration. These conditions range from almost complete anoxia in ponds covered with ice and at very low temperatures (Blazka, 1958), to conditions in summer when temperatures in excess of 25°C and supersaturation with oxygen are not uncommon (Jones, 1961;Garey & Rahn, 1970). Thus it is of interest to study the respiratory physiology of this fish to establish the mechanisms which enable such wideranging adaptation to occur. An interaction between temperature and respiratory processes is inevitable not only because of the direct effects of temperature on metabolic rate but also because of its marked influence on the oxygen content of water inspired by the fish. The main purpose of the present investigation was to obtain baseline information regarding the normal respiratory and blood characteristics of carp at two different temperatures, and to measure changes in routine metabolism of fish acclimated to these temperatures during exposure to lowered oxygen content of the water. Several previous investigations of this kind have been carried out but few have used a flow-through respirometer together with recent methods for cannulation of the dorsal aorta for blood sampling (Beamish, 1964;Garey, 1970).

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