A laterally averaged, one-dimensional P- and S-wave velocity structure is obtained for Silent Canyon Caldera, located beneath Pahute Mesa at the Nevada Test Site. The velocity models are derived from a linearized damped least-squares travel-time inversion for slowness gradient perturbations. A total of 72 P- and 20 S-wave travel times generated from 13 different explosions, obtained at locations above the caldera, are used in the inversion. The structure is modeled using linear velocity gradients in a series of horizontal layers. The inversion produces a model in which velocity for P waves increases from 1.5 km/sec at the surface to 4.5 km/sec at 2.3 km depth and from 1.0 km/sec at the surface to 2.3 km/sec at 2.0 km depth for S waves. The P- and S-wave models produce travel-time residuals which are typically less than 0.1 and 0.2 sec, respectively, which is within observational error. In an attempt to estimate the degree of lateral velocity variations over which the one-dimensional model averages, waveform data collected above the caldera are examined for indications of a laterally heterogeneous structure.