Constitutive models in soil mechanics are used to determine theoretical solutions to a host of geotechnical engineering problems such as pile-soil interaction, pile installation, short and long term stability, consolidation and foundation problems, etc. A fundamental component of the numerical formulation of the constitutive equations is the development of the elastoplastic tangent stiffness matrix. The numerical formulation of the elastoplastic matrix for a work hardening/softening material, as applicable to a general purpose finite element computer code, has been attempted by the author. The derived tangent stiffness matrix is symmetric for the associated flow rule (g = f) but it is non-symmetric for the non-associated flow rule (g ≠ f). All the components of the elastoplastic matrix for the Lade's model\sp[11] are also given. A parametric study was carried out to determine the validity of the associated and the non-associated flow rules for various stress paths in the deviatoric plane. The results demonstrate the inability of the associated flow rule to capture the soil behavior, except at stress levels very close to the hydrostatic axis.