New data are presented for major dissolved species in vent fluids from the TAG hydrothermal mound on the Mid-Atlantic Ridge, sampled in 1993, 1994, and 1995. The latter two cruises bracket the drilling of the mound by the Ocean Drilling Program in September--November 1994. Changes in venting patterns have been observed over this time period, both before and after drilling. However the major ion chemistry of venting fluids appears to be stable, indicating that the drilling did not penetrate the primary plumbing system of the mound. ¿ American Geophysical Union 1996