Accumulation of manganese along the East Pacific Rise is much too rapid to be explained by simple authigenic precipitation. Since the rise crest sediments are oxidizing throughout the entire, though short, sedimentary column, it is unlikely that the high manganese accumulation rates represent upward flux of manganese during diagenesis. The most probable source of this excess manganese is hydrothermal activity. By assuming that the accumulation rate of hydrothermal manganese along mid-ocean ridge crests is proportional to the area of new crust formed per year, it is possible to estimate that 9 × 1011 g/yr of manganese is produced by hydrothermal activity at spreading centers. Data on manganese loss from basalt due to hydrothermal leaching and on manganese concentrations in hydrothermal solutions support this estimate. Hydrothermal manganese flux to the oceans is about three times higher than the dissolved load of manganese carried by rivers (2.5 × 1011 g/yr). Thus, hydrothermal manganese introduced at oceanic spreading centers should be considered one of the major sources of manganese input to the oceans.