North Pacific deep-sea sediments recovered by Ocean Drilling Leg 145 constrain the timing of the late Pliocene major intensification of Northern Hemisphere continental glaciation. A change in sediment character from nonglacial to glacial--specifically, abrupt increases in the amount of terrigenous clastic sediment and ice-rafted debris--occurred all across the North Pacific at about 2.67 Ma. This change occurred very rapidly, perhaps within 2000 yr, too quickly to be a direct response to either tectonic or orbital forcing. As the number and thickness of volcanic ash layers increased 10-fold at this same time, we suggest that rapid intensification of glaciation may be associated with the widespread episode of explosive volcanism that began then.