This paper records an experiment to determine the transfer-function of a human operator, when he is acting as an element in a closed-loop control-system, by a mothod of analysis which is based on correlation functions. The theoretical analysis shows that under certain conditions the transfer-function of the operator can be defined by an integral equation that relates a, cross-correlation function of his response to an observed ‘ error ’, to the auto-correlation function of tluit ‘ error ’. It is shown how this integral equation can be solved approximately by a technique which uses an electronic analogue-computer. In this instance such a computer has been used to determine the parameters that will ‘ best fit ’ an assumed form of transfer-function to the recorded data.