A complete description is given of a respirometer, suitable for unrestrained five to ten gram bats, from which the following variables were simultaneously directly measured: air and body temperatures, relative humidity, oxygen consumption, carbon dioxide production, water loss, and electrocardiograms. Temperature was measured with specially mounted bead thermistors and a wheatstone bridge. Relative humidity was measured with an adsorptive electronic sensor which was excited by a transistorized oscillator, and had the voltage output rectified and fed to an oscillograph. Electrocardiograms were taken with custom-made safety pins chronically indwelling through the dorsolateral thoracic epidermis. Electrical connection was made with common clothing snap fasteners. Various controlled humidities were produced by bubbling air through saturated salt solutions. Water vapor was converted to acetylene and collected along with respiratory gases over mercury for subsequent analysis with a gas chromatograph
