As part of the Joint Oceanographic Institutions Deep Earth Sampling project, the interstital waters of cores from 69 holes were sampled for deuterium analysis. Sixteen of the cores penetrated sediments as old as Eocene, and several sampled Cretaceous sediments, which allowed us to examine changes in the deuterium content of the oceans with time. Deuterium is shown to be a conservative constituent of the pore water. Its abundance in the pore fluids can be changed by diffusion, but the rate is slow, and corrections for this effect have been made. Changes in the aboundance of deuterium can be related changes in the amount of ice stored in continental glaciers, inasmuch as precipitation in the form of snow is highly depleted in deuterium compared with the oceans. Many of the cores show a change in isotopic composition of samples from early to late Miocene that can be ascribed to the buildup of the Antarctic ice sheets. After correcting for the role of diffusion in reducing the isotopic contrast between samples from a single core, we estimate an increase of 10 per mil (%) ΔD (corresponding to a Δ18O change of about 1.2%) between the early and late Miocene. A similar analysis of Pleistocene to Holocene changes indicates a ΔD rise of 8% during the time of maximum continental ice, which corresponds to a Δ18O increase of about 1.0%. On the basis of limited data, we find no ΔD change in the oceans from Cretaceous to Miocene.