Electromagnetic catheter blood-flow meters in current use can be considered to be modifications and adaptations of electro-magnetic velometers developed in 1944 for local flow velocity studies in physical systems. Availability of materials like Teflon and polyethylene which can be introduced into the blood stream without causing rapid clotting stimulated development of intravascular flow sensors in competition with extravascular electro-magnetic flow meters. The intravascular flow meters can be subdivided into two classes: 1) external-field flow meters, where the magnet is external to the animal or patient, and where only the electrodes are introduced into the blood vessel, and 2) "self-contained" flow meters, where the magnet is introduced into the blood stream together with appropriately disposed electrodes. The intravascular devices may be either velometers, which measure the local velocity of flow in a blood vessel, or volume rate of flow meters. In one type of velometer the conventional scheme is turned inside out. The flow being measured passes around the flow sensor rather than through it. Either the magnetic field may be derived from a flat coreless coil, or the radial component of a field generated by a cylindrical iron-core magnet may be used with adjacent electrodes separated by a septum. The conventional scheme of passing the flow through the transducer lumen may also be used in catheter flow meters. Instruments of this type can be classified as I) transverse-flow catheter flow meters, or 2) longitudinal flow sensors. In type 1) catheters, the flow-sensing element incorporated into the catheter has a lumen whose cylinder axis is perpendicular to the catheter axis. They are thus sensitive to flows which are at right angles to the catheter axis. In type 2) catheters, the lumen is coaxial with the catheter so that flows parallel to the catheter axis are measured at the location of the sensor. Both types of catheters may be either used as volume rate of flow meters, by placement against the ostium of an artery so that all of the flow entering it must first pass through the flow sensor, or used as velometers, by placing the flow sensor into the lumen of a major blood vessel with the lumen axis of the transducer parallel to the blood vessel axis. Some designs do not have provisions for centering and angular alignment of the velometer in the blood vessel, while others do. In addition to the magnetic field of a flat coil and a substantially radial magnetic field, the field between two parallel wires carrying opposite currents can be used to generate the intravascular magnetic field. This configuration permits the design of a collapsible flow sensor which can be passed through a narrow tube (such as a needle or catheter passed through the skin and underlying tissues into an artery), and which will expand in the artery to touch diametrically opposed points of its wall. This flow sensor also serves as an artery gage, giving its diameter through radiography. This knowledge permits calculation of the volume rate of flow after establishment of the average flow velocity by velometry. The greatest degree of miniaturization of intravascular flow sensors is achieved through the use of an external magnetic field. Only little more than electrodes and their lead wires need be incorporated into the catheter; it can thus be small enough in diameter to pass through a French-5 catheter which can be used for percutaneous introduction of the flow sensor into the vascular tree.