A physiological sensor-transmitter assembly for measuring a physiological property at an internal body location of a subject over time is described. An instrument for inserting the assembly in the subject and a method of monitoring a physiological property measured at the internal body location over time are also described. The assembly includes a sensor and a transmitter adapted to transmit information from the sensor in a non-wired fashion to a receiver. The assembly also includes an anchor adapted to attach to an externally accessible portion of a subject and a tether that connects the sensor and the anchor to maintain a position of the sensor within the gastrointestinal tract of the subject.