The flow rate of sea water through sub-sea-floor hydrothermal systems at mid-ocean ridges has been estimated at $$1.3-9 \times 10^{17} g/yr$$ by consideration of the rate at which circulating fluid must advect heat out of the spreading plates into the oceans. The rate of hydrothermal heat advection was obtained by computing the difference ($$40 \pm 4 \times 10^{18} cal/yr$$) between the theoretical heat production associated with sea-floor spreading and observed heat flow measurements. Effects of exothermic chemical reactions, direct heat loss from flows extruded on the ocean floor, and heat of crystallization of basalt were minor and yielded insignificant contributions to the estimate. The majority of dredged ocean-floor metamorphic rocks appear to represent the products of intensive hydrothermal reaction with hot sea water. The chemical trends (loss of Ca and K, gain of Mg, Na, and $$H_{2}O$$) and alteration mineral assemblages of these rocks closely resemble those observed in both laboratory alteration...