Excessive application of swine manure to a field over long durations can increase nitrate-nitrogen (NO 3 -N) leaching as a result of accumulation of more nutrients in the root zone than the subsequent crops may need. The objective of this study was to use the GLEAMS (V.2.1) model to compare measured versus simulated effects of swine manure application with urea-ammonium-nitrate (UAN) on subsurface drain water quality from beneath long-term corn (Zea mays L.) and soybean (Glycine max L.) plots. Four years (1993-1996) of field data from an Iowa site were used for model calibration and validation. The SCS curve number and effective rooting depth were adjusted to minimize the difference between simulated percolation below the root zone and measured subsurface drain flows. Model predictions of percolation water below the root zone followed the pattern of measured drain flow data, giving an average difference of 10%, and –5% between predicted and measured values for manured and UAN-fertilized plots, respectively, for four years from 1993 to 1996. Model simulations for overall NO 3 -N losses with percolation water were comparable to measured NO 3 -N losses with subsurface drain water giving an average difference of 20% for manured plots. The model overpredicted NO 3 -N losses, particularly for soybean on plots, which received manure in the previous year. Predicted NO 3 -N losses with subsurface drainage from fertilized plots were much lower than measured values with an average difference of –32%. The best fit line with zero intercept showed correlation coefficients of 0.73 and 0.66 between monthly predicted and measured NO 3 -N losses with subsurface drain flows for manured and UAN-fertilized plots for four years from 1993 to 1996, respectively. The results of the study show that the N-transformation processes and the associated rate factors based on soil temperature and soil water levels may need to be refined for consistent simulation of NO 3 -N losses with subsurface drainage water when fertilized with either swine manure or UAN for corn production.