The reaction rates of plutonium-239 and boron (1/v) were measured as a function of space and time in a large block of beryllium oxide following the injection of a pulse of fast neutrons. The time-dependent reaction rates associated with the various Fourier spatial modes were derived from the experimental results. The experimental value of the peaking time of the plutonium-239 reaction rate associated with the fundamental spatial mode was found to be 19.85 + 1.1 μ sec. A theoretical value of 16.5 μ sec was obtained using a time-dependent zero-dimensional diffusion code and a crystal scattering kernel for beryllium oxide. It is in marked disagreement with the experimental value. Measurements made of the time to peak in the plutonium-239 reaction rate as a function of distance from the source showed a total variation of a 4 μ sec in peaking time over a distance of 53 cm. Theoretical calculations of the peaking time as a function of distance showed a similar variation and the general trends were in good agreement with the experimental result. There was, however, a constant discrepancy between the theoretical and experimental results of the order of that noted for the fundamental mode.