Thermodynamics of isotropic polymer solutions with anisotropic molecules modelled as extensible ellipsoids of revolutions is considered. Anisotropy of molecules is accounted for by introducing internal variables - structure and elongation tensors describing re-orientation of the molecules and their elongation. Decomposition of the thermodynamic fluxes into dissipative and elastic constituents is assumed. The thermodynamic analysis leads to a constitutive law for the stress tenser and evolution equations for the structure and elongation tensors. Study of elongational flow shows how different relations among elastic moduli lead to different extents of the re-orientation and elongation of the molecules. Modelling of simple flows shows that the coupling between the structure and elongation tensors can lead to an overshoot in one of them even in a start-up of elongational flow. The effect on the stress, however, is less pronounced.