Seismograms from three nuclear explosions recorded along a 2400-km long deep seismic sounding profile extending from the Yamal Peninsula to Lake Baikal are presented. We have constructed a preliminary 1-D compressional velocity model to fit the major travel time features of the data. Clear differences between the seismograms from each shot imply that the velocity structure of the crust and upper mantle is variable along the profile. Estimated crustal thicknesses range from 47 km beneath the west Siberian rift to 42 km beneath the Siberian craton. The structure of the upper mantle above the transition zone consists of material with P-wave velocities ranging from 8.25 km/s at the Moho to 8.53 km/s at 225 km. Velocities increase abruptly at 225-km depth of 8.63 km/s. There is little evidence for a pronounced low-velocity zone, although small velocity reversals are required to terminate several branches of the travel time curve. The upper-mantle transition zone discontinuity at 425-km depth consists of a 5.6% first-order velocity increase. The lower discontinuity has a 4.9% velocity increase over the depth range 656.5 to 659 km. ¿ American Geophysical Union 1993