Experience in 1993 as well as many other years has clearly shown that unpredictable, but frequently occurring heavy rains will result in significant N loss through denitrification or leaching. In those years, farmers and their advisers often wonder how much loss has occurred and whether or not it would be beneficial to apply more N late in the season. In order to answer that question, one needs to first be able to predict a) how much of the applied N was present as N03-N, b) what portion of that N was lost, and c) whether or not it would be profitable to apply supplemental N late in the season. Over the last 12-15 years, a series of studies have been conducted in Illinois and Iowa with the ultimate goal of finding techniques to use in predicting the consequences of N loss.