Hydraulic fracturing experiments at two underground and one near-surface location in igneous and shale formations were described. The tests were designed to study the feasibility of hydraulic fracturing as a method of determining in-situ stresses. The tests were carried out in open holes of 2-3/8-in. diameter. Fracturing tests on two 5-ft diameter cores were also reported. The test results revealed an increase in the magnitude of the stress as the face of an opening was approached from inside a rock mass. Horizontal fractures also were observed in areas of reportedly high lateral stress, providing some evidence for the validity of the providing some evidence for the validity of the principle of least resistance. The results also principle of least resistance. The results also indicate that caution must be used in using the shut-in pressure as a measure of the least compressive stress. Introduction: Hydraulic fracturing is best known as a well stimulation method. There are other important applications, however, for which the process shows great potential. One of these is in the area of in-situ stress determination as suggested by Scheidegger Kehle and Fairhurst. The mechanics of the fracturing process is the same in any application, and improvement of the method may therefore be expected through a mutual exchange of experience in each of these areas. The theory of the hydraulic fracturing technique relates measurable quantities such as the breakdown pressure and the instantaneous shut-in pressure to pressure and the instantaneous shut-in pressure to the tectonic stresses and certain physical rock properties. properties. Assuming negligible pore pressure and fluid penetration, the break-down pressure (pC) at the penetration, the break-down pressure (pC) at the instant of fracture initiation is given by the following expressions....................(1) when the fracture extends in a "radial" direction (in a plane parallel to the axis of the borehole). And...................(2) when the fracture extends in a direction normal to the borehole axis. Corresponding expressions that include the effect of pore pressure and fluid penetration are given in the literature Because our work was done in dry and impermeable formations, Eqs. 1 and 2 are considered adequate. These formulae are based on the assumption that the borehole is drilled parallel to 3 and that the rock behaves as a linearly elastic isotropic material; it also assumes that the fracture is initiated in a direction perpendicular to the least compressive stress, i.e., 2 or 3, respectively, in accordance with the principle of least resistance. The terms "radial" and "normal" fractures are introduced in place of the commonly used terms "vertical" and "horizontal" fractures in order to avoid possible confusion in the event a borehole is drilled in a direction other than the vertical.