Many types of apparatus have been described for determining basal metabolism of small animals (1-4), especially the rat. Haldane's open circuit method(5), in which oxygen consumption and carbon dioxide production are determined simultaneously, has been widely used with various modifications. Most of such equipment has been designed for research purposes and is too complex and expensive for routine use in student laboratory experiments when several sets of the apparatus are required.The device described in this paper is economical, simple to construct and operate, and quite accurate for equipment of its simplicity. About 30 pieces of this apparatus have been in routine use in our laboratory during the last 3 years with good results.The apparatus is shown schematically in Fig. 1. A standard wide-mouth, half-gallon Mason jar is secured to a wood base by 2 wire straps. The glass liner is removed from a zinc top∗ and a 13 mm hole drilled in the center. A 5 ml burette is made by etching lines on an 11 mm o.d. g...