Estimates of the heat output of hydrothermal vents, identified along the Endeavor and Southern segments of the Juan de Fuca Ridge, are used to evaluate the total heat flux associated with hydrothermal circulation for the ridge segment. A 50-m array carried by DSV Alvin sampled the temperature and vertical velocity structure of hydrothermal plumes from individual vents. These measurements are used to estimate the thermal flux associated with such plumes. The maximum heat flux calculated for a single vent is 50 MW (1 MW=1¿106 W). The median heat flux per vent is 9 MW and 3 MW, respectively, for the Endeavour Segment (18 vents) and Southern Segment (18 vents). The estimates for any given vent may vary over an order of magnitude. This uncertainty is due mainly to the difficulty of locating the centerline of the plume relative to the point of measurement, although the uncertainties in the constants for the appropriate equations based on laboratory experiments also contribute significantly to the net error. For the Endeavor Segment, the minimum total geothermal heat flux due to hydrothermal circulation exceeds 70 MW. The minimum estimate for the Southern Segment is 16 MW. The maximum estimate is probably closer to the total heat flux from high-temperature venting (239 MW and 66 MW respectively). High-temperature hydrothermal venting accounts for only a small fraction of the heat available according to steady state predictions of conductive heat flux; other hydrothermal phenomena (e.g., diffuse flow) probably account for a greater proportion of the total hydrothermal heat flux. ¿ American Geophysical Union 1993