Abstract A method is presented allowing direct and continuous estimation of the excitability changes of single fiber terminal arborizations within the central nervous system. In essence, the method measures the current required to maintain a preset antidronic firing probability of the unit under study. This implies operation in a closed loop system controlled by a computer. With this technique one can accurately determine input-output curves of single units requiring relatively small current variations (0.15–2.1 μA) to change from zero to maximal probability of response. The method also allows measurement of the excitability changes produced by conditioning volleys to sensory nerves.