The clinical information available from ultrasonic Doppler blood velocimeters may be analysed in several ways, spectral analysis probably being the best. This is, however, a relatively expensive technique when compared with the cost of commerically available Doppler velocimeters. A large part of the diagnostic information present can be obtained from the maximum blood velocity in the vessel at any one time. A relatively inexpensive analogue method of extracting this information from the Doppler signal is described, and the preliminary results obtained on signals derived from peripheral arteries are discussed.