A young (<1 m.y.) tholeiitic basalt dredged from the Mid-Atlantic Ridge displays a 234U excess and a 230Th deficiency that have resulted from the addition of seawater uranium during weathering at seafloor temperatures. Two older samples, though they acquired substantial amounts of uranium from seawater, are depleted in 234U, indicating preferential leaching of this isotope. Hydrothermally altered samples suggest that some uranium loss may have occurred. Possible isotopic effects of preferential 234U leaching, however, are obscured by secondary addition of seawater uranium at lower temperatures. ?? 1978.