As more experience is gained with the use of ultrasonic techniques for diagnostic purposes, there is a need to improve the quantification of the data obtained from these instruments. In particular, the ability to study discrete areas within the body with range-gated Doppler systems now offers the potential to obtain data on ventricular size and performance as well as imaging other cardiac geometrics. However, a serious deficiency exists in the inability to accurately locate the sample-volume of interest and the direction of the sound beam within the body. While the slant-range from the transducer to the sample-volume is available, there is presently no adequate, real-time method of determining its position with respect to either an external or patient reference system. It is therefore proposed to develop a technique and actual system to meet this need. A sonic locating system employing two small spark-gaps on the axis of the ultrasonic transducer will be used to establish a line in space. The location of the two points in space and the three-dimensional, algebraic equation for the transducer axis will be generated by an inexpensive, dedicated microcomputer. Inclusion of the range information will allow for accurate location of the sample- volume with respect to any reference system. It will then be possible to make measurements of various cardiac geometrics and determine flow directions and velocities.