Salt rocks, such as potash and rock salt, show a complex mechanical behavior that is rate and history dependent. Various approaches exist to represent the main features of the response of these rocks through constitutive models. The approach proposed in this paper is to formulate the constitutive equations using internal state variables (ISV) in the context of the unified theory of inelastic flow. To illustrate the application of this approach, the authors use an updated version of a viscoplastic model with reduced expressions for laboratory triaxial tests at relatively high confining pressure. It is shown that, under these conditions, mixed hardening of salt rocks can be described through a combination of scalar and directional ISV. The authors also describe in some detail a procedure to obtain the material parameters for the model equations. The paper shows some characteristics of the model response for various loading paths, illustrating the good agreement with experimental observations and highlighting some of the differences with a classical isotropic hardening model.