Critical experiments have been performed on a mock-up of the compact nuclear power source (CNPS), a small reactor system designed to provide power at sites where fuel costs and logistics make fossil-fuel-powered systems less attractive. Although the program has been canceled, its unique physics characteristics make the CNPS a useful benchmark for medium-enriched uranium-graphite-moderated reactors. The physical design, details of the critical experiments, and the methods and results of the analysis are described. The discrepancies between calculations and experiments are such that, though further modeling work is necessary to delineate the causes, the beginning-of-life performance of the reactor was adequately predicted.