All of the analyzed igneous rocks from DSDP (Deep-Sea Drilling Project) legs 34 and 37 are altered. Their primary oxygen isotope composition, which can be estimated from the analysis of separated minerals, was indistinguishable from that of fresh unaltered basalts dredged from midocean ridges (5.7¿0.2% SMOW), but the alteration of the rocks has either enriched or depleted them of 18O. Low-temperature weathering of basalts by seawater has increased their 18O content by 1 to 3%0. Basalts recovered from 600 m within the ofceanic crust in site 332B have a Δ18O value o 8.30/00. On the other hand, the 18O contents of intrusive rocks from site 334 have been lowered 1 to 3000 by high-temperture, postsolidus exchanges with seawater. Both kinds of altered rocks are already known from the study of dredged materials, but only the ocean drilling program can determine the depth to which seawater penetrates in the oceanic crust and the prevalence of altered rock. Implicit in current tectonic models is the hypothesis that sufficiently large columes of oceanic crust are produced each year, so that any appreciable exchange between the crust and the oceans will influence the chemical composition of seawater. If site 332B has sampled typical oceanic crust, then weathering of the crust is indeed a major sink for 18O. However, if intrusives are prevalent in the crust, then their alteration is a major source of 18O to the ocean. Estimates of the volumes of materials involved in the formtion and alteration of the oceanic crust (1 km of hydrothermally altered rock, 600 m of weathered rock), cycling of water through the mantle (2% of the subducted crust), and weathering of the continents indicate that the isotropic composition of the ocean may be held at its present value as a consequency of the 18 enrichments balancing the 18O depletions. Approach to the present isotrope ratio ratio is governed by a time constant of 200 or 300 m.y.