We have mapped the distribution and intensity of submarine hydrothermal emissions from the summit caldera of Axial Volcano by means of hydrographic and chemical sampling of the water column during annual cruises in four consecutive years (1985--1988). These investigations were undertaken to ascertain the strength of hydrothermal venting relative to other vent fields on the Juan de Fuca Ridge and to gauge the importance of Axial Volcano as a source of hydrothermal emissions to the northeast Pacific Ocean. Measurable temperature anomalies are mostly restricted to a 200-km-thick water column within the caldera, where they range from a background level of ~0.01 ¿C to local maxima of 0.02--0.1 ¿C above known high-temperature vent fields. The temperature anomaly plume is significantly smaller in both extent and intensity than plumes emitted by vent fields on other segments of the Juan de Fuca Ridge. A maximum estimate of the total heat flux from the summit, based on the total excess heat and advective velocity of the plume, is 8¿108 W. Hydrothermal venting seems surprisingly small for an edifice that testifies to a long-term oversupply of magma at a single axial location. We speculate that additional heat may be lost from Axial Volcano by diffuse percolation of seawater over broad areas of its flanks and by magma eruption and venting along flank rift zones. ¿ American Geophysical Union 1990