  We discuss the evolution of gravitational waves in a brane-world cosmology
embedded in five-dimensional anti-de Sitter spacetime. We show that during
slow-roll inflation, modelled as a period of quasi-de Sitter expansion on the
brane, there is a discrete normalizable massless graviton mode. There is a mass
gap due to the expansion, above which there is a continuum of massive modes.
Only the massless mode is `light' compared with the Hubble scale during
inflation, leading to the production of classical perturbations on large scales
from vacuum fluctuations on small scales. We calculate the amplitude of these
fluctuations at horizon-crossing and show that the standard four-dimensional
result is recovered at low energies, but the amplitude of the perturbations is
enhanced at high energies.
