An invasive, fluid velocity measuring wire guide particularly adapted and of a size (less than 0.030 inch) for subselective placement in the coronary arterial tree, which includes a Doppler mechanism for determining the blood flow velocity in the region of the distal end of the wire guide. Although the wire guide is particularly adapted for diagnosing coronary arterial disease subselectively; it is of such a size and manipulability to be useful in other blood flow and biological fluid flow analyses. Preferably, the wire guide if flexible and steerable for precise placement. The wire guide includes an elongated wire member having a Doppler crystal attached in longitudinal alignment at its distal end. Electrical leads extend from the Doppler crystal along the wire member to the appropriate test equipment for measuring the fluid flow velocity in the region of the Doppler crystal.