A method of formant tracking is described in which the speech wave is passed through an adjustable network whose transfer function approximates to the inverse of the vocal tract filter. From the output of this network it is possible to derive error signals which indicate the polarity of the unbalance for each formant frequency. These signals can be used for continuously improving the approximation so that the zeros in the inverse network continuously follow the poles, or formants, in the vocal tract filter. A test setup, using analog computer type elements, is described, and its performance examined both with synthetic input and with real speech input.