Fifteen heat-flow values, representing the southernmost measurements made in the Atlantic, show a pattern of heat flow with tectonic features around the ridge-fracture-fracture triple junction which are explained by effects of hydrothermal circulation. High values (>4 &mgr;cal/cm2sec) on ridges which flank the axial valleys may either reflect a topographically controlled ascending limb of a hydrothermal circulation system, or result from conductive cooling of crust subsequent to the sealing of cracks generated at the spreading center. Intermediate to low values (0.4 to 1.8) in the depressions of fracture zones and axial valleys are consistent with a model of thermal cracking and subsequent hydrothermal circulation in the newly formed rocks. Low values (<1.3) over the ridge flanks may also be a consequence of secondary circulation in cracks re-opened during dehydration of rocks previously hydrated near the spreading centers. If such secondary circulation is widespread, it would imply at least two, and perhaps more, minima in profiles of oceanic heat-flow vs. age.