This volume covers Leg 61 of the cruises of the Drilling Vessel Glomar Challenger. Leg 61, from Guam to Majuro, Marshall Islands, was planned to concentrate on the Mesozoic (Jurassic and Cretaceous) history of the oldest portion of the Pacific Ocean deep-sea floor. Our goals were to study the paleontologic, sedimentary, petrologic, tectonic, and magnetic histories of that area from Recent to Late Jurassic time, by drilling a deep re-entry site into the Nauru Basin west of the Ralik Chain of the Marshall Islands. This area formed at a fast-spreading Pacific Plate boundary 145 to 155 m.y. ago, in the Late Jurassic. Cores from this locale should allow us to better understand biostratigraphic evolution and sedimentary processes in a Mesozoic open-ocean environment, the petrologic nature of fast-spread oceanic crust, the tectonic history of the Late Jurassic Pacific Plate, and the nature of the Jurassic magnetic quiet zone. D/V Glomar Challenger departed Guam on May 22, 1978, to begin Leg 61. The leg terminated at Majuro, Marshall Islands on July 29, 1978.