A thermodynamic analysis of tokamak plasmas is developed, starting from a single fluid model. Some general properties of the entropy production in the plasma are discussed. An approach is employed which is more general than the one based on Onsager symmetry relationships. This choice appears to be justified by experiments. The concept of "thermodynamic stability" is introduced, and a general criterion for the stability of stationary states far from thermodynamic equilibrium is assessed. An upper limit on the particle density is found to exist as a necessary condition in order to prevent disruptions. Analysis of the entropy balance shows that this limit is modified when neutral beam heating is applied. Radial temperature profiles are shown to be stable against external heating under specific experimental conditions, in particular when ion and electron temperatures are comparable. An offset scaling law for the energy confinement time tE = tE(Pinp) (with Pinp total input power) is obtained for stationary states, when stability against additional heating is imposed.