Drifter data from the Gulf of Mexico are used to assess and enhance the output of a primitive equation general circulation model. The analysis is made in a 450 km¿450 km open subdomain encompassing a Loop Current ring. The model velocity field is compared with position data from four drifters at the drogue depth of 50 m using geometrical orthogonal functions (GOF). An Eulerian velocity field is reconstructed from the model velocity field and drifter velocities. This reconstructed velocity improves 8-day numerical trajectories relative to the model field by at least an order of magnitude, as quantified by two Lagrangian error metrics referenced to the real drifter paths. An Eulerian metric that compares the two fields, however, does not exceed 7% for the 20-day assessment period. Thus the drifter data may be reproduced with modest impact on the model velocity. Enhancement of the model velocity field is determined by two tests: the ability of the GOF velocity field to (1) improve the forecast of drifter positions using only a posteriori data and (2) improve the forecast of withheld drifter data. Using a posteriori data, the 20-day temporal mean of the position error is improved for all drifters by 87--89% for 6-hour and 26--38% for 30-hour forecasts. For 6 days, a withheld drifter is 35--40 km from a drifter whose velocity is used in the reconstructed velocity field. The temporal mean of the position error during this period is improved by 20% for 6-hour and 26% for 30-hour forecasts. ¿ 2001 American Geophysical Union