Permeability measurements of samples of the Topopah Spring Member of the Paintbrush Tuff and the Bullfrog Member of the Crater Flat Tuff collected from the Nevada Test Site have been made at room temperature and in a temperature gradient. The room temperature permeabilities of the samples ranged from 0.8¿10-18 to 64¿10-18 m2 for the Topopah Spring Member and 0.5¿10-18 to 8.4¿10-18 m2 for the Bullfrog Member. These variable permeabilities probably reflect differences in crack and void configurations among the samples. The permeability of a given sample was only slightly affected by heating to maximum temperatures between 90¿C during experiments of up to 5 weeks' duration. The observed lack of permeability change is consistent with the hypothesis that the disposal of nuclear waste in tuff units in the unsaturated zone at the Nevada Test Site would not have a large effect on the permeability of the host rock. The fluids discharged from the tuff samples during the experiments were mostly dilute solutioins of nearly neutral pH, whose compositions differed only slightly from the starting J13 groundwater composition. Waters of this type should be relatively noncorrosive to stainless steel waste canisters.