An ambulatory physiological evaluation system including a main and a background gamma radiation detector provided in a vestlike garment worn by a patient for the purpose of monitoring and/or diagnosing the patient's physiological activities during a prescribed period of time. Also included is an apparatus and method for determining the exact location of the left ventricle of the heart, positioning the main radiation detector relative to the left ventricle, and maintaining the position of the main radiation detector relative to the left ventricle during an ambulatory study period. The ambulatory physiological evaluation system also contains electronic circuitry which monitors and processes information obtained from the radiation detectors. Information from the main and auxiliary detectors, as well as ECG electrodes is recorded on a portable cassette recorder. After the information has been recorded over a desired period of time, the information is presented, through an interface and an analog to digital converter, to the memory of a stand alone computer located in a hospital or office. The computer calculates such items as R-R time intervals, electrocardiagram and time-activity curves, and displays these items and other physiological data for both the main and auxiliary detectors. From the calculations made by the computer, average heart rate, number of aberrant beats, left ventricular ejection fraction and relative cardiac blood volume and other values of physiological significance may be calculated for a time interval of interest.