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The characterization of input-output relationships in reponse to small perturbations is a basic approach to the experimental study of a variety of systems. These exist numerous instances where, in addition to convenience, messurements should be performed in a minimum time. This papeg describes a method whereby a pseudo-random binary signal, properly filtered and synchronised, is used in conjunction with fast digital Fourier transform techniques to yield rapid transfer function estimates. Its implementation results in a novel instrument which is well adapted to real-time on-line applications. Evaluation in the course of studies on electrical characteristics of nerve membrane have confirmed its rather unique combination of speed, dynamic range, accuracy, and flexibility of operation.