The Guaymas Basin, Gulf of California, is a hydrothermally active sediment-covered ridge crest. At present, it is the best studied of this type of ridge system. The methods that have been used are extensive deep and shallow drilling and coring, recovery of hydrothermal deposits via dredging and submersible, and sampling of the hydrothermal fluids as well as the recognition of the chemical anomalies these fluids create in the waters of the basin as a whole. Hydrothermal activity has also been observed in the sedimented Escanaba Trough of the Gorda Ridge and Middle Valley of the Juan de Fuca Ridge, but less sampling has been done at these sites and hydrothermal fluids have yet to be recovered from Middle Valley. The deposits at all three sites show many similarities in spit of the differences in the sediment composition in the three areas. Data from experimental interaction of sediment and seawater are helping to clarify the way in which reactions occurring in the sediment cover affect the chemistry of the hydrothermal fluids. Although isotopic data on the Guaymas Basin fluids suggest that they have reacted with underlying basalt, new experimental data show that the water chemistry can be closely reproduced by reaction of sediment and seawater alone at temperatures and pressures appropriate to the Guaymas Basin system. This suggests that the hydrothermal fluids in the experiment and in the basin have equilibrated with the sediments, and that the relative importance of reactions with basalt versus those with sediment as a source for most of the dissolved sp cies in the hydrothermal fluids cannot be discerned unless a distinctive isotopic signature is present.

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