The Root Zone Water Quality Model (RZWQM) was previously modified to simulate subsurface drain flows and evaluate the impact of different tillage systems on subsurface drain flows (Singh and Kanwar, 1994). This article discusses further modifications made in the RZWQM to simulate nitrate-nitrogen (NO3-N) concentrations and NO3-N losses with subsurface drain flows. Daily NO3-N concentrations were simulated in subsurface drain flows under four different tillage systems: chisel plow (CP), moldboard plow (MB), no-tillage (NT), and ridge-tillage (RT) by using the modified RZWQM. Simulations were conducted for the growing seasons of three years (1990 to 1992). Simulated NO3-N concentrations and losses with subsurface drain flows were compared with the measured data obtained from a water quality research site at Nashua, Iowa. Predicted NO3-N concentrations generally followed the same pattern as the observed concentrations. Simulated annual average NO3-N concentrations in subsurface drain flows were within 11% (averaged over all three years) of observed annual average NO3-N concentrations in subsurface drain flows. The model correctly predicted maximum concentrations under MB treatment and minimum under NT for all three years. Simulated annual NO3-N losses were within 14% (averaged over all three years) of observed annual NO3-N losses. Various NO3-N transformation processes need to be calibrated as a function of tillage system to improve model performance.