The delta 18 O values of the wall rocks in the vicinity of the Tayoltita mine were shifted to significantly lower values as a result of interaction with geothermal fluids of predominantly meteoric origin. The areas of strongest 18 O depletion (delta 18 O rock < 4%) form a north-northwest-south-southeast trend coinciding with the areas of Au-Ag mineralization in the vein system. At a detailed level, however, there is only minor isotopic evidence for interaction between the vein fluids and the wall rocks. This finding is confirmed by the oxygen isotope compositions of mineral separates from the rocks and quartz and calcite from the veins. Chlorite, epidote, and feldspar in the wall rock have delta 18 O values too high to have formed in equilibrium with the same fluids that formed the vein minerals. Further, except for chlorite close to major vein structures (< or =2 m), the delta 18 O values of neither chlorite nor epidote were significantly shifted by interaction with the later fluids. The delta 18 O values of feldspar do appear to have partially equilibrated with the later fluids. By estimating the delta 18 O values of the evolving fluids from analyses of the other minerals, we have calculated that the degree of oxygen isotope shift of the feldspars is consistent with a period of fluid circulation lasting 10,000 to 100,000 years at a Darcy flow velocity of 10 (super -9) to 10 (super -11) m/s. This indicates that although pervasive fluid flow through the rocks may have occurred during the formation of the veins, it was extremely limited and had little effect on the regional delta 18 O rock patterns. The coincidence of the veins with the areas of strong 18 O depletion of the rocks must, therefore, be an indication that there was a single, long-lived episode of hydrothermal activity that remained focused in the same vicinity throughout the life of the system.