A method was developed and investigated for directly measuring the shutdown margin in a reactor operating at a subcritical steady state. Measurements were made on the Iowa State University UTR-10 reactor with conditions ranging from almost critical to fully shutdown. The method is based on the fact that the frequency dependent transfer function of the reactor is a measure of the response of the reactor to an input disturbance. The magnitude of the transfer function of a subcritical reactor depends on the frequency of the disturbance and the steady-state multiplication factor. Since the multiplication factor is a measure of the reactivity, the amplitude of the transfer function is a direct indication of the shutdown margin. A sinusoidal disturbance was introduced by means of an oscillator operating in the center of the core. The oscillator was operated at a fixed frequency of 10 cps in order to eliminate the frequency dependence. The fluctuation was measured by a BF3 detector located against the outside of the core opposite the oscillator. The signal emitted by the detector contained bombardment noise and instrument noise in addition to the desired information. The signal was passed through a band-pass filter and was then cross-correlated with a pure sine wave to obtain the transfer function. A plot of the transfer function vs. the multiplication factor was a smooth function from which the multiplication factor could be found with a confidence of Â± 0.04% near critical and with a confidence of Â± 0.17% in the fully shutdown condition.