A closed-circuit system for the simultaneous measurement of oxygen consumption in six small laboratory animals is described. The system consisted of six glass chambers in a big water bath of constant temperature, an oxygen-supply unit, six pressure sensors, a microcomputer and a printer, or, optionally, a desk-top calculator. Ventilation within the chambers provided rapid equilibration of air temperature and humidity conditions. Easily sealing glass lids facilitated operation and maintenance of the system. Carbon dioxide was absorbed inside the chambers by soda lime. At 6-min intervals the consumed oxygen in each chamber was automatically replaced with oxygen from a low-pressure tank. The microcomputer, a self-constructed piece of equipment containing a Z 80 A central processing unit, received pressure data from the separately operating sensors, determined the decrements in chamber pressure due to oxygen consumption by the animal sitting in the chamber, and stored the calculated values in a 2 Kbyte RAM for outprint at the user's request. The apparatus proved to be sensitive, accurate, and easy to operate.