The accurate measurement of a forced vital capacity (PVC) maneuver in a small mammal [mouse, rat, guinea pig, hamster] requires that the plethysmograph in use have good response characteristics at high frequencies. On a theoretical basis, the behavior of zero-order (pressure type), 1st-order (flow type without inertance) and 2nd-order (flow type with inertance) plethysmographs are developed. The actual frequency response of a mouse-sized plethysmograph is presented and a technique for improving its response characteristics is described. A flat amplitude response (within 3% of a reference flow taken as truth) was obtained for sinusoidal inputs below 240 Hz. A phase lag with respect to the reference flow equivalent to a simple time delay of 1.4 ms was observed up to 150 Hz. A typical FVC curve for a mouse is shown, and criteria are provided for designing similar plethysmographs suitable for use with larger animals.