The effect of rock plasticity in hydraulic fracturing has generally been ignored. The importance of plasticity in shales has been recognized by Perkins, who considered effects in competent rock unimportant. All the Perkins, who considered effects in competent rock unimportant. All the commonly used theories of fracture propagation treat reservoir rocks as ideally elastic materials. Even surface energy effects in crack extension have been neglected except in some early work by Perkins and Bartlett and Perkins and Krech. Probably the chief reason for ignoring these phenomena is that they are not easily incorporated into the theoretical phenomena is that they are not easily incorporated into the theoretical approaches previously used. The Lagrangian method, which is presented in an earlier paper, provides a means of including plasticity and surface energy in fracturing provides a means of including plasticity and surface energy in fracturing theory. However, to apply it we must have a basic understanding of plastic behavior in reservoir rock under tension. The literature contains little information on this subject. Essentially, all the work on plastic deformation of rocks has been done in compression. This paper presents experimental results from both field and laboratory work to provide at least a start in understanding the tensile plastic properties of rock materials. We have used these results along plastic properties of rock materials. We have used these results along with the Lagrangian method to evaluate the practical importance of plasticity in conventional fracturing operations. Our conclusions are less plasticity in conventional fracturing operations. Our conclusions are less than definitive. It appears that plasticity is important in some, but not many, reservoirs of modest depth. As fracturing methods are extended to greater depths and more unconventional types of rock formations, however, we can expect plasticity effects to become more important. Therefore, refinement of the theory and collection of additional experimental data are clearly worthwhile.