This analysis of polymer rheology uses conditional probability distributions to describe the phase space dynamics of all macromolecules in a polymer melt. The result is a viscoplastic constitutive equation for the polymer stress. Using conditional probability distributions makes the use of a large number of bead-spring chains in the modeling system possible, but precludes evaluating the intermolecular contribution to the total stress. Both the kinetic and intramolecular contributions are evaluated for a system composed of an arbitrary number of bead-spring chains that interact with one another using molecular dispersion forces. The analysis predicts that the kinetic contribution is isotropic and the intramolecular contribution is viscoplastic. The intermolecular contribution is assumed negligible in comparison to the intramolecular contribution because it results from physical bonds among chains, while the intramolecular contribution results from chemical bonds within a chain. © 1993 John Wiley & Sons, Inc.