Abstract: Apparatus for locating blood vessels in a live human body is self-contained, compact, non-invasive, and fully movable about the body.Within a housing of the general size and shape of a pen and having a distal end and a proximal end, solid state electrical circuitry generates high-frequency electrical signals producing ultrasonic signals directed ahead of the distal end toward a narrow area of a body, receives the ultrasonic echoes of the ultrasonic signals and generates high-frequency electrical signals in response to said echoes, mixes both high-frequency electrical signals and amplifies the resulting differentials to produce beats, the amplitudes of which will vary if a Doppler effect due to the pulsating flow of blood through a vein or artery is detected. These amplified differential signals or beats are used to drive a miniature loudspeaker at the proximal end of the housing.