A simple apparatus is described which utilizes a slightly modified Mason jar as a container in which a small animal, such as a rat, may be placed for the measurement of O2 consumption. The animal is immobilized by a small wire mesh cage. CO2 is absorbed with soda lime and O2 consumption is measured by timing the progress of a soap film along a calibrated glass tube attached to the Zn top of the jar. Data obtained by students are presented which indicate that even relatively small effects of drugs on basal metabolism may be detd. with this apparatus.