The response of ocean circulation to the shoaling of the Central American Seaway (CAS) is investigated using the Massachusetts Institute of Technology (MIT) Ocean General Circulation Model (OGCM). In contrast to earlier model studies, it is found that significant amounts of deep water are formed in the North Atlantic prior to the closure of the CAS. However, the circulation pattern is fundamentally different from the modern ocean. In the upper layers of the CAS, there is a relatively strong geostrophic flow from the Pacific to the Atlantic, controlled by the pressure difference across the seaway. However, when the CAS is deeper than the level of North Atlantic Deep Water (NADW) outflow, a significant amount of NADW passes through the CAS to the Pacific Ocean. In the Pacific, the deep water traverses the basin from east to west in a relatively narrow zonal jet, and becomes a southward flowing western boundary current, before it joins with the Antarctic Circumpolar Current (ACC) to the south. This implies that deep sea sediment records from the Miocene Pacific Ocean could have been influenced by relatively young NADW and provides a new framework for the interpretation of geochemical tracer data. Diversification of benthic foraminifer fauna suggests that the CAS shoaled to a depth of about 1000 m toward the end of the middle Miocene. This would have prevented the passage of NADW to the Pacific and established the modern deep water circulation pattern at that time.