The increasing concerns regarding the quality of precipitation falling on the United States resulted in the formation of the National Atmospheric Deposition Program (NADP) in 1978. The first phase of the program, which was the development of a nationwide monitoring network, was organized in July 1978 by the North Central Region of the State Agricultural Experiment Stations. Currently, 90 sites across the country are participating in the collection of wet and dry deposition samples that are sent to a Central Analytical Laboratory (CAL) located at the Illinois State Water Survey. Site operators are required to determine pH and specific conductance before shipping samples to the CAL. Upon arrival, samples are analyzed for eleven parameters that include pH and specific conductance. To ensure reliable analytical results both at the field sites and at the CAL, a quality assurance program has evolved that includes intersite comparisons for pH and specific conductance, computer verification of all analytical results, plus the extensive use of "blind" samples. Details of the internal controls used at the CAL are documented with supporting data from the first three years of network operations.