Abstract: An apparatus, system, and method enable monitoring of one or more physiological conditions in an animal including pH. A monitor may take the form of a bolus that may be installed in a stomach of an animal such as in a reticulorumen of a ruminant animal. The monitor may include sensors for detecting one or more of the pH, temperature, pressure, and position. The monitor includes a power source and electronics for processing, storing, and transmitting signals representing the physiological conditions detected by the sensor(s). The monitor may include first and second portions that are releasably joined, and that may support respective components for ease of assembly during manufacture and servicing. The monitor may form part of a system having a receiver for receiving and interpreting the signals from the monitor. The method includes installing the monitor in an animal and monitoring the animal by detecting physiological characteristics.