Closed-circuit or confinement chambers for measuring oxygen consumption of small animals can be constructed mainly from standard laboratory ware. The system described by Smothers (1966) is a good example. Alternatively some degree of automation may be added (e.g. Heusner et al., 1971; Stock, 1975). The classical gravimetric method of Haldane (1892) combined with a modern electronic balance has still much to commend it. All of these chambers may be easily assembled without resort to expensive equipment, but the animal is confined and cannot readily be handled; and measurements are restricted to collection periods of short duration. Whilst these methods are valuable in some circumstances, they should be used with an awareness of their limitations and the results obtained with them interpreted with caution. For example, our experience is that scaling up a few short measurement periods does not give an accurate estimation of 24 h energy expenditure; and in addition the animals may take some time to adapt to their new housing and measurements over this period may be misleading.

For fast response measurements, particularly on humans and large animals, and for long-term studies, an open-circuit system employing flowmeters, electronic gas analysers and recording equipment is necessary. Various combinations of equipment are possible depending on the degree of accuracy required and the finance available. In this chapter we describe a few complete systems that have been found to operate satisfactorily, and which could be adopted in part or in entirety by the reader.