We report the results of CaSiO3 perovskite-melt partitioning experiments conducted at conditions relevant to the uppermost lower mantle. Partition coefficients, measured by LA-ICPMS, show that CaSiO3 perovskite has a strong affinity for REE, Y, Th and U, making CaSiO3 the most likely host for these elements in the lower mantle. Comparison with the low-pressure analogue CaTiO3 shows that large ions partition similarly into the Ca-sites of the two perovskites. Thus CaTiO3 may be used as a model to investigate the factors controlling the partitioning of many trace ions into CaSiO3. Combined fractionation of MgSiO3 and CaSiO3 perovskites during an early magma ocean has been suggested as a means of enriching the lower mantle in silica. Our results show, however, that the chondritic upper mantle ratios of refractory lithophile elements would be lost after ~17% of perovskite fractionation. A large perovskitic layer `hidden' in the lower mantle is therefore improbable.