Results of a comprehensive series of quasi-static triaxial compression tests on a South African norite tested at confining stresses of up to 8,000 p.s.i. are presented. The tests were performed in a stiff testing machine so that the post-failure behavior could be observed, and lateral volumetric strains were continuously recorded using the "constant confining stress" technique. A comparison of the results of these tests with some data previously presented for a Witwatersrand quartzite reveals that there are significant differences between the deformational behavior of norite and quartzite, particularly in the manner in which the lateral expansion at the maximum axial stresses varies with confining stress. For norite, the amount of lateral expansion in excess of that due to linear, elastic compression was found to steadily increase about threefold between confining stresses of 50 and 6,000 p.s.i., and thereafter remain approximately constant. The "excess lateral expansion" for quartzite is about four times higher at a confining stress of 2,500 p.s.i. than at 50 and 5,000 p.s.i. The rate of lateral expansion at the maximum axial stress decreases with increased confinement in a similar manner for both rocks, although at any particular confining stress the absolute rate is higher for quartzite than for norite.