A specially designed confining pressure vessel is described that allows tensile samples to be tested under a superimposed confining hydrostatic pressure. Tests on samples of well characterized materials such as aluminum were used to verify the operation of the system, calibration of the internal load cell, and data reduction methods. The results of a series of exploratory tests done on the inert material 900-24 are described. 4 refs., 17 figs., 2 tabs.