To investigate neutron flux time dependence in fast pulsed assemblies that have little or no regeneration, two semianalytic models have been developed and tested successfully against accurate but not transparent calculations. The models have been developed through the use of analytic time dependence and numerical space and energy dependence by the use of time moments. Comparison of the models has been made to exact two- and three-group diffusion theory. An interpretation of the parameters of the semianalytic model has been given in terms of standard nuclear parameters.