We attribute the evolution of deepwater circulation during the late middle Eocene and Oligocene to the opening of circum-Antarctic gateways at the Drake Passage and Tasman Rise. These gateways allowed the development of a proto-Antarctic Circumpolar Current (proto-ACC) by the late Eocene, and the subsequent thermal differentiation of Southern relative to Northern Component Water (SCW vs. NCW). The timing and influence of the tectonic events are important in understanding their significance for deepwater flow during this time period. Shallow flow through the Drake Passage began by approximately 41 Ma; the Tasman Rise experienced an initial minor deepening from approximately 39-36 Ma, possibly allowing the formation of a shallow proto-ACC. Further deepening of these gateways during the early Oligocene resulted in intensification of flow, with a full depth ACC established in the late Oligocene. We compare benthic foraminiferal stable isotope records from Deep Sea Drilling Project (DSDP) Site 366 with published records from other sites to reconstruct deepwater circulation in the eastern equatorial Atlantic from the late middle Eocene to earliest Oligocene ( approximately 40-33 Ma). Comparison of Site 366 delta (super 18) O values to Southern Ocean Site 689 during the late middle Eocene indicates that SCW was the dominant water mass in the eastern equatorial Atlantic at the time of initial flow through the Drake Passage. In the early late Eocene, delta (super 18) O records from Site 366 and equatorial Pacific Site 1218 indicate that, in place of SCW, a warmer deepwater mass was dominant in the equatorial ocean in both basins. Site 366 delta (super 18) O values indicate a shift back to SCW in the middle late Eocene, likely as a result of enhanced SCW production. During the latest Eocene, Site 366 delta (super 18) O values diverged from Site 689, indicating a return of warmer deepwater during this time, possibly due to deepening of the proto-ACC and resultant reduction in SCW export.