Because the lifetime of and heat extraction rate from a hot dry rock (HDR) geothermal reservoir can be substantially controlled by the in situ rock thermal conductivity, information concerning the dependence of thermal conductivity on moisture content and temperature is important for proper design and management of the reservoir. Results of thermal conductivity measurements are given for 14 drill core rock samples taken from two exploratory HDR geothermal wellbores (maximum depth of 2929 m (9608 ft) drilled into Precambrian granitic rock in the Jemez Mountains of northern New Mexico. These samples have been petrographically characterized and in general represent fresh competent Precambrian material of deep origin. Thermal conductivities, modal analyses, and densities are given for all core samples studied under dry and wet ('water-saturated') conditions. A cut bar thermal conductivity comparator and a transient needle probe were used for the determinations with fused quartz and Pyroceram 9606 as the stardards. The maximum temperature range of the measurements was from the ice point to 250¿C. The measurements on wet rock were limited to the temperature range below room temperature. Conductivity values of the dense core rock samples were generally within the range from 2 to 2.9 W/mK at 200¿C. Excellent agreement was achieved between these laboratory measurements of thermal conductivity and those obtained by in situ measurements run in the HDR wellbores. By using samples of sufficient thickness to provide a statistically representative heat flow path, no difference between conductivity values and their temperature coefficients for orthogonal directions (heat flow parallel or perpendicular to core axis) was observed. This isotropic behavior was even found for highly foliated gneissic specimens. Estimates of thermal conductivity based on a dispersion and/or mixture analysis utilizing pure minerallic phase conductivities and detailed modal analyses usually agreed to within 9% of the experimental values.