Heat flow measurements have been made in five deep crustal holes drilled into the mid-Atlantic ridge by the Deep-Sea Drilling Project ship Glamar Challenger. The data provide new information on the hydrothermal circulation that probably controls the crustal temperature distribution near ocean ridge spreading centers. Nearly constant very low heat flow of 0.6¿0.1 μcal cm-2 s-1 (25¿4 mW m2) was found for three deep holes and three standard ocean probe measurements across one 3-km-wide sediment pond on 3.5-m.y.-old sea floor and to a depth of 400 m into the underlying basaltic basement. The heat flow predicted by conductive cooling models of of place accretion is much higher, about 6.4 μcal cm-2 s-1 (270 mW m-2). The result suggests that heat is carried to the surface by extensive hydrothermal circulation that extends at least to the lower part of layer 2, such that the heat flux based on conductive estimates represents only a fraction of the total flux. A borchole into 16-m.y.-old sea floor appears to have penetrated a convective system. Temperature measurements in the basement part indicate that seawater was flowing rapidly down the hole into a peameable horizon near the bottom at 324-m depth. The heat flow of 0.5 μcal cm-2 s-1(21 mW m-2) measured in the overlying sediments suggests that before the hole was drilled, water at about 10¿C may have been flowing horizontally in this horizon.