The timing of events leading to the earliest connection between the Pacific and Atlantic oceans at Drake Passage is controversial but important, because gateway opening probably had a profound effect on global circulation and climate. A rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the South American and Antarctic plates at about 50 Ma, from N-S to WNW-ESE, accompanied by an eightfold increase in separation rate. This would have led to crustal extension and thinning, and perhaps the opening of small oceanic basins, with the probable formation of a shallow (< 1000 m) gateway during the Middle Eocene. No change in South American-Antarctic motion is observed near the Eocene-Oligocene boundary, but a deep-water connection developed between 34 and 30 Ma, when continued extension led to the initiation of seafloor spreading at the West Scotia Ridge. These timings correlate with events seen in the oxygen isotope record from benthic foraminera, and support the view that Drake Passage opening was the trigger for abrupt Eocene-Oligocene climate deterioration and the growth of extensive Antarctic ice sheets.