Heat flow measurements in the eastern Pacific now total over 800, a sufficient number to permit the analysis of their distribution within a wide range of age zones. Major results are that (1) the midocean ridge crestal regions have lower heat flow than that expected from conductive cooling models of the lithospheric plate, (2) an increase in heat flow is observed some distance away from the crest in a zone which appears to delineate a transition from dominantly convective to conductive heat transfer, and (3) the heat flow in crust older than this transition zone closely approximates that predicted by conductive cooling models for oceanic lithospheric plates. The transition from low to high heat flow occurs when 150-200 m of sediment is deposited over basement, apparently preventing convective transfer of heat from the oceanic crust to seawater. Thus the general shape of the empirical heat flow versus age curve for the East Pacific Rise (EPR) agrees with that from the Galapagos Spreading Center, but the age of the transition zone occurs in 5- to 6-m.y.-old crust on the Galapagos Spreading Center, whereas it occurs in 10- too 15-m.y.-old sea floor on the EPR. Proximity to the equatorial high sedimentation region causes the deposition of thick layer of sediment much more quickly on the Galapagos Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topographic relief.