Fluid in inclusions in four samples of quartz from the Noche Buena mine, Zacatecas, Mexico, has been analyzed by means of a neutron activation technique. The samples were irradiated prior to extraction of the fluid from the inclusions in order to minimize contamination. The concentration of sodium in the inclusion fluids ranged from 0.8 to 1.8 percent, and their manganese content ranged from 50 to 170 ppm. Neither copper nor zinc was detected. Studies of the detection limits of our analytical techniques indicated that the concentration of copper was less than 12 to 24 ppm and that of zinc less than 260 to 580 ppm in all of the inclusion fluids. The inclusion fluids analyzed were almost certainly saturated with respect to chalcopyrite and sphalerite at the time of entrapment. Therefore, at least some hydrothermal solutions in equilibrium with these minerals contain only small concentrations of copper and zinc.