This volume covers Leg 45 of the cruises of the Drilling Vessel Glomar Challenger. Leg 45 of the Deep Sea Drilling Project was the inaugural cruise of the International Phase of Ocean Drilling. Its primary scientific objective was to develop an understanding of the composition, structure, and origin of oceanic crust by drilling as deeply into it as technology would permit. Legs 45 and 46 were conceived as a combined attempt either to drill a single deep hole into North Atlantic oceanic crust, using, if possible, all the time of both cruises, or, in the event of early loss of a deep hole, to drill holes in several surveyed areas in a roughly east-west transect, out to crust about 110 million years old. The areas of principal interest for Leg 45 were survey areas AT-5, situated near 23 °N, 46 °W, and AT-6, near 23 °N, 43°W. The two survey areas are on opposite sides of the Mid-Atlantic Ridge, south of the Kane fracture zone in the vicinity of magnetic anomalies 4, 4', and 5. Both regions were generated along approximately the same segment of the Mid-Atlantic Ridge between 6 and 13 million years ago, and have since been separated by sea-floor spreading. The drilling on Legs 45 and 46 was only the initial stage of a longer term program aimed at determining the petrologic, geochemical, magnetic, physical, and structural properties of the crust beneath the ocean floor, to obtain an understanding of the processes which produce these properties, and to outline the evolution of oceanic crust through time. D/V Glomar Challenger left San Juan, Puerto Rico, on 30 November 1975 to being Leg 45, and completed its voyage 52 days later in San Juan, on 20 January 1976. Three holes were drilled, two in survey area AT-5, and one in survey area AT-6, all in middle to upper Miocene crust about 150 km on either side of the Mid-Atlantic Ridge (Site 395, west of ridge axis, latitude 22°45.35'N, longitude 46°04.90'W; Site 396, east of ridge axis, latitude 22°58.88 'N, longitude 43°30.95 'W). The specific problems we were trying to solve on Leg 45 can be divided into three general categories: 1) Chemical stratigraphy and petrogenesis of lavas, 2) Structure and origin of layer 2, and 3) Modification of the crust during and after cooling.