Stable isotope records spanning from the latest Oligocene through the middle Miocene have been produced at Sites 999 and 1000 to address major changes in climate associated with the transition from relative global warmth of the late Oligocene-early Miocene to the Neogene "ice-house" world. The isotopic records of Sites 999 and 1000 clearly show the middle Miocene oxygen-isotope increase from approximately 15.5 to 13 Ma, and all Miocene oxygen-isotope events "Mi" can be recognized. Additional delta (super 18) O increases occur at approximately 24.5, approximately 23, 20.5, and 17.8 Ma. In addition, the long-term delta (super 18) O increase between approximately 15.5 and 13 Ma shows internal quasi-cyclic structures, reflecting a periodicity of approximately 400 k.y., the orbital eccentricity, which is superimposed on the longer term cooling trend related to the major icesheet growth in Antarctica. The long-term trends in carbon isotopes are marked by two positive excursions: the delta (super 13) C increase near the Oligocene/Miocene boundary, from approximately 24.5 to 23 Ma, and the Monterey Excursion between approximately 17 and 13.5 Ma. The increase in delta (super 13) C at 17 Ma leads the first major cooling step in Antarctica by 1.5 m.y. However, within the Monterey Excursion several delta (super 13) C maxima occur with cyclic periods of approximately 400 k.y. in parallel to climate excursions as indicated by oxygen isotopes. Long-term isotope shifts, coinciding with lithologic changes indicating the changes in sediment physical properties, are linked to changes in climate.