A sand transport model using White's <1979> sand flux equation and the Mars general circulation model <Pollack et al., 1990> was developed to understand the erosional sources, transport pathways, and depositional sinks of windblown sand on Mars. An initially uniform distribution of sand (4 mm over the entire surface) is regionally transported based on wind stress, saltation threshold, and percentage of topographic trapping. Results are consistent with the observed polar and Hellespontus dunes and Christensen's <1986> modeled block size distribution, but only for an extremely low saltation threshold (0.024 N/m2). Low thresholds generally result in transport of sand-sized particles originating in the northern mid latitudes to the north pole, and transport from the northern lower latitudes to the southern hemisphere. Our results indicate that the polar dune fields could form in 50,000 years, consistent with the active polar dunes and lack of longitudinal dunes observed on the surface of Mars. ¿ 1999 American Geophysical Union