Paleobiologic evidence indicates that the Early-Late Maastrichtian boundary was marked by major changes in the marine realm, including the extinction of inoceramids, a sharp decrease in the diversity and abundance of ammonites and the loss of Caribbean-Tethyan rudistid reef ecosystems. A global synthesis of Maastrichtian stable isotope, lithologic and faunal assemblage data, including new isotope data from two DSDP cores in the north and south Atlantic, provides insight into oceanographic conditions associated with mid-Maastrichtian extinctions. Stable isotope records from Atlantic, Tethyan and Indian Ocean sites exhibit slight increases in delta (super 18) O and delta (super 13) C values in Lower Maastrichtian sections, relatively constant values in Upper Maastrichtian sections and suggest that oceanographic conditions at these sites were largely invariant throughout much of Maastrichtian time. By contrast, Lower Maastrichtian sections from Southern Ocean sites exhibit large, parallel excursions in the carbon and oxygen isotopic compositions of benthic and planktonic taxa, including 1 to 2 per mil negative shifts in delta (super 13) C and 1 per mil positive shifts in delta (super 18) O values. These excursions begin just above the Campanian - Maastrichtian boundary (base of C32N1) and end near the Lower-Upper Maastrichtian boundary (upper part of C31R). Isotope records from Upper Maastrichtian Southern Ocean sections are similar to those from Atlantic, Tethyan and Indian Ocean sites. Considered in conjunction with lithologic and faunal assemblage data, isotope records are interpreted to reflect the progressive isolation of water masses and local upwelling at southern high latitudes during the Early Maastrichtian, followed by a restoration of communication with the global oceanic circulation system near the Early-Late Maastrichtian boundary. Isolation of Southern Ocean water masses may be related to a eustatic fall in sea level, which roughly coincides with the base of the excursion in delta (super 18) O and delta (super 13) C values at southern high-latitude sites. As such, it is evident that mid-Maastrichtian circulation changes were local in extent, restricted to the southern high latitudes and had little influence on conditions in the Atlantic, Tethyan and Indian ocean basins. In this context, changes in the marine realm that led to the extinction of a wide range of shallow water, nektonic and benthic taxa remain unexplained.