Free-air gravity and topography data from the British Isles have been analysed in the frequency domain to determine the mechanisms of topographic support. First, we calculate how the admittance between measured free-air gravity and topography varies as a function of wavelength. A simple model, which consists of a crust with two layers of different densities overlying a higher density mantle, was then used to calculate admittance functions. The misfit between observed and calculated admittance was minimized by varying elastic thickness and the proportion of the total load due to internal loading. At short wavelengths λ < 200 km), the fit between observed and calculated admittance is consistent with an elastic thickness of 5 ± 2 km and a small proportion of internal loading (c. 0.2). The general behaviour of the observed admittance suggests that topography is primarily supported by elastic stresses in the crust. A small departure between observed and calculated admittance at the longest wavelengths (400< λ <1000 km) is consistent with modest dynamic support which is probably convective in origin.