Deep water circulation patterns for the middle Eocene to middle Miocene were reconstructed by using Nd values of fossil fish teeth recovered from three deep sea drill cores in the Atlantic sector of the Southern Ocean, ODP Sites 689,690 and 1090. Nd values of Paleogene water masses were used as end-members to evaluate water mass mixing. Secular variability observed at the resolution of these records (50-300 kyr) is primarily attributed to changes in the relative contributions of different water mass end-members in response to tectonic and climatic conditions. At 41 Ma all three of the sites demonstrate a shift toward radiogenic Nd values that appears to be a regionally synchronous event in response to the initial opening of Drake Passage, which introduced radiogenic Pacific sea water into the Atlantic sector of the Southern Ocean. Subsequent shifts toward more radiogenic Nd values at Site 689 (37 Ma) and 1090 (28.5 Ma and 22 Ma) reflect increased throughflow of Pacific sea water into the Atlantic sector, resulting from the progressive widening and deepening of Drake Passage. An intermediate depth connection between the Pacific and Atlantic Oceans in the late Eocene is inferred from the Nd isotope data. This conclusion is corroborated by evidence for increases in phytoplankton production in the Atlantic sector during the late Eocene. (modif. auth. abstr.)