Current measurements and conductivity-temperature-depth surveys of the lagoon and ocean at Majuro Atoll, the Republic of the Marshall Islands, were made from January 10 to 24, 1997. A vertically integrated tidal current model reproduced qualitatively well tidal ellipses calculated from the observed current measurements by Fourier transform. A three-dimensional, robust diagnostic residual current model explained the major features observed in the current measurements averaged over the dominant tidal cycles. We used the diagnostic model to examine the effects of wind stress, radiation stress, density gradients, and tidal stress on the exchange of water between the lagoon and the ocean. Wind effectively mixes the lagoon water in approximately 2 weeks. Tidal flushing appears to be restricted to a small area near the main channels connecting the lagoon to the ocean. Cross-reef-flat currents induced by radiation stress and flowing to the deep channels in the center of the northern boundary form the dominant mechanism for exchange between the lagoon and the open ocean, causing water to exchange completely with the ocean in about 15 days. Computer-generated particles tracked through the lagoon showed that radiation stress is also the main forcing mechanism for particle export from the lagoon. However, the coupling of tidal exchange through the Calalin Channel and wind-stress-induced mixing in the lagoon could also provide a significant export mechanism, particularly for particles originating uniformly inside the lagoon. ¿ 1999 American Geophysical Union