New experimental techniques are presented for the measurement of pore pressure as an additional dependent variable under undrained triaxial test conditions. Experimental data are presented for the variation of pore pressure in Kayenta sandstone at various degrees of water saturation. We find that the following are in good agreement with theoretical predictions: (1) the rate of increase of pore pressure pw with externally applied pressure p, (2) the compressibility of fully saturated samples, and (3) the volume strains at which pore pressure begins to build up. The strength of samples taken to failure is found to be a unique function of p-pw; i.e., the effective stress law is found to hold for this material.