This article is concerned with what are considered to be the significant feedback mechanisms of EBR-I Mark III. The objective is that of providing an explanation of the dynamic behavior of this particular fast reactor. A mathematical model of the core and blankets is postulated and an analog of the equations is constructed. The response of the model and of the reactor to the same signal at any given operating conditions are in good agreement. The analog facilitates an analysis of the feedback producing the response. The reactor is considered to constitute a closed loop nonlinear mechanical system with forcing functions resulting from variations in neutron density and the flow of NaK through the core and blankets. The significant sources of internal feedback are considered to be the variation in volume of the uranium and the variation in the density of NaK. Resistance to the free motion of uranium in response to thermal expansion provides the significant nonlinearities in the system. This resistance results from the physical characteristics of the redundant structure constituting the core, blankets, and containing shell. All the equations defining the time dependent physical phenomena are developed from an analysis of the reactor system, but the constants in the nonlinear equations of motion of the materials of the core and blankets are synthesized from low power operation of the reactor.