A whole body plethysmograph for pulmonary measurements of unrestrained small animals includes a cylindrical animal chamber and an integral reference chamber. The animal chamber and the reference chamber each include ports for receiving transducer nipples. The animal chamber and the reference chambers also include ports for selectively venting each of the chambers. A port is mounted in a bulkhead which separates the animal chamber and the reference chamber, and a length of tubing extends from the valve into the reference chamber for providing a high resistance bleed path between the animal chamber and the reference chamber. The plethysmograph further includes an aerosol manifold in the top of the animal chamber. The aerosol manifold includes a plurality of radially extending apertures for uniformly distributing the aerosol in the animal chamber. The plethysmograph still further includes two pneumotachographs in the top of the animal chamber. The plethysmograph may be used either as a volume device or a flow device. However, the preferred method of use is as a flow device.