The general program established at Argonne National Laboratory in connection with the Fast Power Breeder Reactor (PBR) includes performance of exponential experiments on assemblies having compositions which may exist in the reactor core. This report deals with that phase of the program known as the Fast Exponential Experiment which may be described very briefly as follows. An assembly of fissile (U-235) and fertile material (too small to be self-critical) intermixed with poisons (such as are encountered in the mechanical structure and coolant system of a self-sustaining reactor) is fed with neutrons from an auxiliary source. By measurements of the neutron flux within the assembly, important parameters can be calculated which are necessary to the proport design of an actual critical reactor of the same composition.