Using diffusion theory and the eigenfunction expansion method, a detailed time-dependent study of fast neutrons has been made for three iron assemblies: infinite, 1 m3, and 0.5 m3. Various results have been obtained by taking two different source energies, namely, 14.47 and 1.0076 MeV. All the calculations have been carried out using the 50-group cross-section set of Ahmed et al. For the 1.0076-MeV source, (a) the time-dependent spectra exhibit two distinct peaks up to ∼50 ns, (b) energy variation of the mean slowing down time shows “anomalous” behavior similar to that observed by Bey non et al., and (c) the most probable time has two different values for some energy groups below ∼240 keV, in conformity with the results of White et al. The double values of (except for the 40th and 41st groups) and the “anomalous” behavior observed in cease when we take the source energy to be 14.47 MeV. Further, for all values of energy Ei, the values of both and decrease as the size of the assembly is reduced. It has been shown that in the two larger assemblies, pseudoasymptotic conditions are established in certain time intervals. It was determined that some energy groups tend to cluster and decay with the same decay constant after a certain time.