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Management of water and fertilizer N are important aspects of cotton production in the desert Southwest. GOSSYM, a cotton growth simulation model, has been used extensively to manage these inputs. Our objectives were to further validate GOSSYM by comparing model-simulated and measured soil NO3--N profiles, to evaluate GOSSYM’s potential as a management tool under irrigated growing conditions in the desert part of the U.S. Cotton Belt, and to address questions about the way GOSSYM simulates NO3--N movement through the soil profile in relation to irrigation water management (which in turn affects prediction of plant growth and development). We compared measured profiles of NO3--N with GOSSYM-simulated profiles. Soil profile samples were obtained from an existing N-management field study, a split-plot within a randomized complete block design. Mainplots were upland and pima cotton (G. hirsutum L. cv. DPL 5415 and G. barbadense L. cv. Pima S-7, respectively). Subplots were a cheek (0 fertilizer N) and three other N-management strategies. The cotton was grown on a Casa Grande sandy loam [fine-loamy, mixed, hyperthermic Typic Natrargid (reclaimed)] near Maricopa, AZ, in 1994 and 1995. Fertilizer N rates ranged from a to 350 kg ha(-1) in 1994 and 0 to 392 hg ha(-1) in 1995. Soil samples taken to a depth of 120 cm in 30-cm increments were analyzed for NO3--N. Comparisons of simulated and actual NO3--N profiles revealed tendencies in GOSSYM to overestimate NO3--N leaching out of the effective rooting zone, resulting in simulated N stresses midseason. When GOSSYM simulated an N stress, between 50 and 75% of the simulated soil NO3--N values were greater than the measured values, yet the simulated N stress still occurred. This indicates possible limitations in GOSSYM’s ability to adequately predict N uptake by plants. The dynamic soil N portion of the model needs further refinement, particularly for cotton production under irrigated desert conditions.