Abstract: An electromechanical monitor for rapid quantitative detection of an anesthetic gas in the gaseous flow of an anesthesia administration system wherein deformation due to directly proportional reversible swelling of an elastic membrane in contact with the gas is mechanically detected and translated into an electrical signal. The membrane is preferably disc-shaped, constructed of silicone rubber, and horizontally mounted within the device. The electrical signal developed may be employed to generate an information display and/or provide automatic control of anesthetic and other gas content within the system.