In order to measure respiration rates in rats irradiated to the whole thorax, a plethysmograph involving a microphone with extended low‐frequency response and an airtight, whole‐body chamber has been constructed. The microphone response to the respiration pressure waves created by the rats placed in the chamber is markedly greater in amplitude than the response to laboratory sounds and animal noises; thereby filtering complexities are avoided. The output from the microphone went to a frequency counter for measurement of the number of pressure waves per unit time. Control, unirradiated rats, between the ages of 3 and 18 months, demonstrated a respiration rate in the range of 130–160 breaths per minute. Rats that received thoracic radiation doses greater than 12 Gy displayed increased respiration rates and occasional abnormal respiration patterns. These results demonstrate that this whole‐body plethysmograph incorporating a sensitive microphone and simple electronics can be used for the measurement of respiration functions in rats with lung damage.