Hydrothermal activity along the global mid-ocean ridge system and at active seamounts introduces a 3He-rich signal into the deep ocean basins, which can be used to trace patterns of ocean circulation and mixing. This hydrothermal helium signal is especially strong in the Pacific Ocean, where the spreading rate of the ridges is the greatest. In several areas of the Pacific the hydrothermal activity is of sufficient strength to produce intense helium plumes which clearly define the regional circulation. Among these are (1) a pair of helium plumes at 2500-m depth, which extend westward from the East Pacific Rise at 10 ¿N and 15 ¿S into the interior of the Pacific basin; (2) a helium plume at 2000-m depth extending southwest from sources on the Juan de Fuca Ridge in the northeast Pacific; and (3) a plume at 1100-m depth emanating from Loihi Seamount at 20 ¿N, which extends eastward from Hawaii to the coast of Mexico. The flow field implied by the helium distribution appears to be in reasonable agreement with Reid's <1997> steric height analysis for the Pacific. ¿ 1998 American Geophysical Union