The time dependence of detector responses in a fissile assembly are usually described by the point reactor model. According to this model the measured prompt neutron behaviour in a subcritical assembly decays exponentially with. a time constant, c which is related to the reactivity of the assembly through. a constant prompt neutron generation time. The present work has shown, through. a series of measurements in two typical fast reactor assemblies VERA 11A and PANDA 1B, that this model is in general deficient. The measured prompt neutron behaviour was not in general described by a single exponential-function, and at high degrees of subcriticality the deviation from ibis type of behaviour was marked. Within 1.5 dollars of delayed critical for VERA 11A the measured prompt neutron behaviour was adequately described by a single exponential function. The resulting prompt neutron decay constants were shown to be consistent with. the reactivity scale derived from a control rod calibration. Time dependent multi—group diffusion theory has been shown to give qualitative agreement with experiment. Detailed discrepancies were at least partly attributable to errors in the macroscopic cross section data used in the calculation.