An ultrasonic Doppler instrument capable of measuring mean and maximum velocities inside a small sample volume is presented. The instrument has been used for determining velocities in normal and diseased heart valves. A method is described for identifying the different heart valves, based on their relation to one another as well as the form of the velocity curve. The pressure drop across the mitral valve can be determined from the maximum velocity and the mean velocity can be used to determine the degree of aortic insufficiency. Recordings of mean and maximum velocities can give an indication of the form of the whole spectrum, thus making complete frequency analysis unnecessary for most purposes.