We evaluated the response of eastern equatorial Pacific upper water column hydrography to the Panama seaway closure and the initiation of large-scale northern hemisphere glaciation. The Δ18O gradient between Globorotalia tumida and Globigerinoides sacculifer indicates a general shoaling of the thermocline between 4.2 and 3.0 Ma which may be related to a recorded decrease in southeast trade wind strength. The Δ13C gradients suggest that surface water nutrient concentrations have increased during the last 5 Myr and that upwelling intermediate waters began to change sources at ~3.2 Ma. Changes in the trans-Pacific east-west temperature and nutrient gradients at 4.0 and 1.5 Ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation. Changes in the sensitivity of surface circulation to Milankovitch forcing, Δ18O records linearly related to orbital variations only after 3.2 Ma, are associated with the seaway closure. Pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to Milankovitch forcing is enhanced by air-sea interactions which maintain the relatively steep Pleistocene trans-Pacific gradients. The Δ13C records are dominated by the 41 kyr period and are usually coherent with high-latitude climate, suggesting that associated changes in the global carbon reservoir dominate the tropical Δ13C signals.¿ 1997 American Geophysical Union