An analysis of the initial neodymium and hafnium isotopic compositions of modern and ancient mantle-derived rocks shows that the divergence of the mantle 143Nd/144Nd and 176Hf/177Hf ratios from the chondritic ratios over the past 2 billion years has been much less than would be expected based on the Sm/Nd and Lu/Hf ratios of the mantle source of mid-ocean ridge basalt. The most likely explanation is that rock material from the continents has been recycled semi-continuously back into the mantle over this time period. The required rate of return of continents to the mantle, calculated from a simple transport model, is about 0.35¿0.15 or the continental mass per billion years (~2.5 km3/year). This result is consistent with the hypothesis that the mass of the continents has been approximately constant since the late Archean and implies that recycled continental material must be an important component of the upper mantle.