A blood oxygen saturation sensor or oximeter is mounted to a probe for insertion into the esophagus of a patient. The sensor includes a set of optical transmission elements and an optical detector for optically detecting the blood oxygen saturation of tissues of the esophagus. By mounting the blood oxygen saturation sensor to a probe for insertion into the esophagus, blood oxygen saturation measurements are obtainable at a physiologically stable site which is fully isolated from ambient light external to the patient. The probe for insertion into the esophagus may additionally be provided with a heart and respiration sound sensor and a temperature sensor. With such a combination, a single probe provides the detection of several useful physiological parameters, including temperature, heart, and respiration sounds and blood oxygen saturation levels. The esophageal probe is reliably and inexpensively constructed of few components and is ideally suited for one-time use.