Electromagnetic flowmeters frequently are used to measure volumetric blood flow in vessels. An in vitro method of calibration, using a constant supply-pressure system, was studied. Potential problems encountered in acute and chronic applications are discussed. Using this calibration system, manufacturers' recommended calibration factors were not in agreement with actual flow calibration factors, and zero, as determined electrically, was not the same as that determined mechanically. Transducer sensitivity decreased with increasing concentrations of red blood cells but was not affected by fluid viscosity. Application problems of vessel constriction by the transducer, transducer orientation about the vessel, and chronic instrumentation are discussed.