Measurements of selected samples of ocean floor basalt indicate that the sources of ridge extrusives for three major spreading centers in the Pacific, Atlantic, and Indian Oceans have a similar range of Nd isotopic compositions. However, there is a marked spread in 143Nd/144Nd ratios within this globally distributed reservior which is evident between oceanic tholeiitic and alkalic basalts as well as between samples of similar chemistry. These differences in 143Nd/144Nd can only be the result of chemical heterogeneities within the ocean ridge basalt (ORB) source that have been maintained for periods which are longer than the lifespan of an active ridge. Basalts from the lau Marginal Basin have 143Nd/144Nd which is marginally lower than that found for ''normal'' ORB. Sm-Nd model ages calculated for primitive oceanic tholeiites suggest a differentiation age of ~1.1 AE for the ORB source, which is similar to estimates obtained from the Rb-Sr and U-Pb systems.