Stable isotope analyses of the benthonic foraminiferal genus Uvigerina from 39 samples at Site 284, covering an interval from the Late Miocene to Early Pleistocene (approximately 6 to 1.5 m.y. BP), are interpreted in terms of glacially induced isotopic changes in the ocean. The Antarctic ice sheet was present throughout this interval. During the Late Miocene its dimensions were less constant than they were during the Pliocene; during the New Zealand Kapitean Stage (Latest Miocene; 4.7 to 4.3 m.y. BP) it was probably substantially greater than today, and a lesser advance may have occurred about 3.5 m.y. ago within the Early Pliocene. Prior to the Kapitean Stage the ice sheet may have been smaller than today, and perhaps less stable. About 2.6 m.y. ago a substantial Northern Hemisphere ice sheet accumulated rather rapidly, and related isotopic and foraminiferal changes mark the base of the Waipipian Stage of New Zealand. A more extensive Northern Hemisphere glaciation occurred during the Early Pleistocene (Hautawan Stage), and was of similar extent to the well-documented glaciations of the later Pleistocene. The oxygen isotopic changes coincide with changes in planktonic foraminiferal faunas considered by Kennett and Vella to represent significant Late Cenozoic paleotemperature changes of surface waters. Carbon isotope analyses also document significant changes associated with these events, the cooling in the Kapitean Stage being particularly marked. We propose the term "Pre-glacial Pliocene" to distinguish that part of the Pliocene prior to about 2.6 m.y. BP, when extensive Northern Hemisphere glaciations had not developed, from the Late Pliocene after development of these ice sheets.