Planktonic foraminifera from DSDP Sites 125 and 132 have been used to monitor the climatic evolution of the eastern and western Mediterranean during the last 5.0 million years. By assuming that members of an evolving lineage have the ecological requirements of the modern end member, a transfer function approach has been used to quantitatively estimate paleotemperatures for this time interval. The paleotemperature records are placed within a biochronologic framework that is based on correlation with previously determined paleomagnetic ages for planktonic foraminiferal events in Pacific sediments. A gradual warming occurs in the Mediterranean throughout the early Pliocene with the exception of a brief cooling during the middle of this interval. A 3-4°C drop in surface temperatures occurs in the early late Pliocene, with cool conditions persisting for a large part of the late Pliocene. The onset of this cooling has been dated at approximately 3.2 m.y.B.P. and appears to be related to the initiation of Northern Hemisphere glaciation. Warm conditions exist across the Pliocene/Pleistocene boundary indicating that this boundary is not marked by a sharp climatic change within the Mediterranean. The early Pleistocene is generally characterized by warmer temperatures that those of the late Pleistocene. In addition, the magnitude of temperature oscillations increases from 2-3°C in the early Pleistocene to 5-6°C in the late Pleistocene suggesting intensification of glacial/ interglacial fluctuations. Both basins are marked by similar temperature conditions throughout the late Pliocene and early Pleistocene indicating a lack of strong east—west trending temperature gradients within the Mediterranean. This changed dramatically in the middle Pleistocene (approximately 1.0 m.y.B.P.) when the western basin (Site 132) became significantly cooler than the eastern basin (Site 125). This change was also coincidental with the intensification of glacial/interglacial oscillations.