The present work involves the design, construction and testing of a device which breaks the EEG into its frequency bands and plots the relative proportion of each frequency over time. Activity is taken from scalp electrodes in the normal fashion and amplified using a high-gain (500,000), low-noise (110 db CMRR) DC instrumentation amplifier. The amplified signal is led to a squaring circuit to provide leading and trailing pulse edges coincident in time with the zero-crossings of the signal. The leading pulse is used to start a precision voltage ramp generator; the trailing edge stops the generator and resets it. The voltage achieved by the ramp is proportional to the time between zero crossings and is taken to correspond to half the period of the current EEG signal. This time is compared to the half period times of the four frequency bands, a counter for the correct band is incremented, and a "band" light is flashed. At the end of a preset sampling time, generally 0.5 to one minute, the contents of the counters are transferred to an Apple computer through a parallel interface and plotted. The resultant graph shows the relative energy in each frequency band over time. The values are also stored on disk for later analysis. The system has been calibrated using a precision frequency generator and is accurate to within plus or minus 0.5 Hz. The total cost of the parts is under $100; a wire wrap schematic is available. Preliminary experiments with the unit have demonstrated definite changes with chiropractic adjustments. It has also been used in a biofeedback mode with the band lights providing feedback as to which frequencies are present.