Seismic-refraction measurements were made along a 600-km profile extending due south from the Canadian border across the Columbia Plateau into eastern Oregon. The source for the seismic waves was a series of 20high-energy chemical explosions detonated by the Canadian government in Greenbush Lake, British Columbia. First arrivals recorded along this profile are on the Pn travel-time branch. In northern Washington and central Oregon, their travel time is described by T = Δ/8.0 + 7.7 sec; but in the Columbia Plateau, the Pn arrivals are as much as 0.9 sec early with respect to this line. An interpretation of these Pn arrivals together with later crustal arrivals suggest that the crust under the Columbia Plateau is thinner by as much as 12 km or has an average P-wave velocity higher by as much as 0.8 km/sec than the 35-km-thick, 6.2-km/sec crust under the granitic-metamorphic terrain of northern Washington. A tentative interpretation of later arrivals recorded beyond 500 km from the shots suggests that a thin 8.4-km/sec horizon at a depth of 100 km may be present in the upper mantle beneath the Columbia Plateau and that this horizon may form the lid to a pronounced low-velocity zone extending to a depth of about 140 km.