An unmoderated chain-reacting system, the ZPR-III zero power, fast critical facility, has been put into operation at the National Reactor Testing Station in Idaho, as part of the Argonne National Laboratory's program to establish the feasibility of the generation of economic electric power by a fast power breeder. The first reactor built in the two-half assembly machine was a rectangular parallelepiped with a critical mass of 142.4 kg of U235 and a core composition roughly that of the proposed Experimental Breeder Reactor II. This was the first of a series of fundamental and applied experiments planned for the facility.