One of the disadvantages of indices derived from ultrasonic Doppler signals is that they are not normally available instantaneously. In an attempt to overcome this problem, a real-time spectrum analyser has been interfaced to an Intel 8085 microprocessor system and programmed to calculate pulsatility index in real time. The only additional patient signal required is a simple three-lead e.c.g. which is used for timing purposes. Although the device has only been in clinical use for a short time it has proved to be very reliable and most useful.