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Two miniature radio transmitters have been developed for telemetering pressure values from internal body cavities. The large sonde has a volume of 4.1 cc and has a lifetime of up to three months when a mercury battery is used. It is employed for physiological studies on animals, and is attached to the wall of the gastrointestinal canal or other body cavities by sutures at operation. The small sonde has a volume of 1.0 cc and a lifetime of three weeks. It is used for gastrointestinal investigations on humans. The transducer in both sondes responds to a pressure variation of 300 cm H20 which gives a 30-Kc deviation of the 300 to 400 Kc carrier frequency.