The use of hydraulic fracturing in elastic rock is described in terms of comprehensive hydraulic fracturing observations in rock salt in the Salado formation in southeastern New Mexico. Inelastic rock behaviour involves both rate-independent and rate-dependent permanent deformations. A combination of small-volume tests in initially open, horizontal drillholes, finite-element analyses, and in situ fracture observations demonstrates that the pressure-time data in inelastic rock can differ fundamentally from thode typically recorded in competent hard rock. Stable pressure-time signatures with little or no pressure drop between peak and driving pressure are obtained in isotropic stress fields. Increasingly unstable records appear to be characteristic for anisotropic stress conditions. Qualitatively, the shapes of the pressure-time records of hydraulic fracturing tests in rock salt appear to be remarkably sensitive to the ratio of the in situ principal stresses normal to the fractures. Obtaining quantitative estimates of in situ stress magnitudes in rock salt and other highly inelastic rocks, however, hinges on the existence of reliable rate-dependent constitutive models in conjunction with relatively complex numerical analyses. Exaggerated contrasts in the greatest and least in situ compressive stresses are inferred from applications of elasticity solutions. Time-dependent effects on the characteristics of hydraulic fracturing in rock salt and associated difficulties in data interpretations arise even if hydraulic fracturing is performed almost immediately after drilling.