Large anticyclonic rings are shed from the retroflecting North Brazil Current (NBC) near 8 ¿N in the tropical western Atlantic. New subsurface velocity and temperature measurements within three such rings are presented here and are found to be consistent with previous in situ and remotely sensed NBC ring measurements. A high-resolution numerical model of the Atlantic Ocean forced by monthly wind stress and an imposed meridional overturning cell is found to shed NBC rings that approximate those observed. The model rings are more surface-intensified than those observed and somewhat smaller in diameter. Both observed and modeled NBC rings move northwestward along the coast of South America with a speed of 8--16 cm/s, considerably slower than predicted by analytical theories describing westward ring propagation. At least 2--3 rings per year separate from the NBC retroflection. Annually, 1--3 rings translate intact from their formation region near 50 ¿W to the islands of the southeastern Caribbean, where they disintegrate after a lifetime of about 100 days. The volume of fluid trapped within the core of an NBC ring and isolated from external mixing is estimated using potential vorticity as a tracer. The horizontal limits of the trapped core volume closely coincide with the radius of maximum swirl velocity, while the vertical limit of the core is typically less than the subsurface extent of significant swirl velocity. The core volume of a typical observed ring is 3.2¿1.0¿1013 m3. This corresponds to an annualized per-ring mass transport near 1 Sv (106 m3/s), similar to previous estimates. This study is the first to make use of subsurface temperature and velocity data to compute the volume of the anomalous ring core. NBC rings may be responsible for 3--4 Sv of direct mass transport across the equatorial-tropical gyre boundary or 20--25% of the total upper ocean cross-gyre transport required by the Atlantic meridional overturning cell. Translating NBC rings may contribute 20% of the total meridional heat transport by the ocean at this latitude. ¿ American Geophysical Union 1995