Flow of the mid-crust in the Tibetan Plateau may strongly influence the patterns of deformation and topography within this area. This flow requires the lower-crust to have low viscosity and so quantifying this viscosity may be used to test the idea of channel flow. An application of Bayesian methods to geologic and geodetic data from north central Tibet yields lower-crustal viscosities (assumed Newtonian) of 1 ¿ 1019 Pa s and 2 ¿ 1021 Pa s (95% bounds). The lower bound is larger than the value of 1016--1018 Pa s required by models of flow of the lower-crust of Tibet, unless the entire lower crust participates in channel flow.