The effects of pressure of up to 10 kb on the electrical resistivity of 30 widely differing crystalline rocks is analyzed. For rocks of low porosity (0.01–0.001), degree of saturation affects both resistivity and the effect of pressure on resistivity. At low pressure the difference in the effect of pressure is particularly marked: a partially saturated rock becomes less resistive, whereas saturated rock becomes more resistive as the pressure increases. Porosity is the sole property which determines the high pressure resistivity of water-saturated rocks composed of nonconducting minerals. Grain size, mineralogy, and degree of alteration, when considered apart from porosity, have almost no effect. Variation in resistivity within one formation or rock type is as great as that among widely different rock types. At pressures above about 3 kb, we found close agreement with two empirical laws connecting resistivity, ρ, pressure, P, and porosity, η: ρrock/ρfluid = η−2 and 1/ρ (dρ/dP) = 0.10 kb−1. With the first relation, resistivity of a rock of known porosity can be calculated relative to fluid resistivity, and with the second, the effect of pressure of up to 10 kb. For a rock composed of conductive minerals, pressure at first decreases resistivity sharply and, then, has almost no effect. For rocks in which pressure causes collapse of pores, resistivity may either increase or decrease with pressure depending on initial connectivity of pores; an increase is the more common. Coarse-grained rocks containing calcite became abnormally resistive at high pressure owing to, we believe, flow of the mineral calcite, with sealing off of interstices. This would suggest the presence of local shearing stress in our hydrostatic experiments. There was no indication of flow in rocks containing dolomite, mica, or serpentine.