This volume covers Leg 90 of the cruises of the Drilling Vessel Glomar Challenger. DSDP Leg 90 successfully obtained a traverse of high-quality middle to upper Cenozoic cores between equatorial and northern subantarctic water masses in the southwest Pacific. Leg 90 is one of several DSDP legs that were set up to complement each other for global paleoceanographic investigations. Others are Leg 68, Legs 72-76, Leg 85 and Leg 87. All were aimed at greater understanding of global paleoceanographic change and thus required continuous sequences exhibiting relatively high sedimentation rates. Seven main sites and one minor site were cored during Leg 90. A ninth site (Site 586) was drilled at the northern extremity of this same traverse during Leg 89. The nine sites are placed along a latitudinal transect to obtain paleoceanographic records for a number of distinct water masses and to maximize our ability to make correlations across the wide latitudes between equatorial and subpolar regions. Scientific investigations planned for Leg 90 materials include the following: 1. High-resolution Neogene biostratigraphy. 2. Latest Paleogene through Neogene paleoclimatic history and surface-water fluctuations in the region using stable isotopes and microfossils. 3. Cross-latitudinal biostratigraphic correlations between sites. 4. Paleobiogeographic change of planktonic foraminifera and other microfossils in relation to paleoceanographic fluctuations and to the northward movement of the Lord Howe Rise during the Neogene. 5. Magnetostratigraphy of the cored sequences to enhance correlation and provide a geochronology. 6. Evolutionary studies of planktonic foraminifera to assist in evaluating different possible processes of evolution, such as punctuated equilibrium versus gradualism. 7. Benthic foraminiferal assemblage changes at these relatively shallow depths during the late Paleogene and Neogene to study paleobiogeography, taxonomic evolution, and the history of vertical water mass changes at intermediate depths in the southwest Pacific. 8. Stable isotopic studies of vertical water mass structure. 9. History of eolian sedimentation related to the development of Australian dry climates and deserts. 10. Physical properties of sediments, including relations with seismic data. 11. Tephrochronology using volcanic ash layers. Leg 90 began in Noumea, New Caledonia in December of 1982 and ended in Wellington, New Zealand in January of 1983.