This paper presents the problem of stability, in the elastic zone, of a freely supported rectangular plate loaded with a local equally distributed load on two opposite sides, which is solved by using an exact in-plane stress distribution throughout the plate. The presented results include plates of form $(\Phi)$ and load cases $(\gamma)$ which are of practical interest. Also given is the solution of the special case, when $\gamma\to0$, which corresponds to the load condition for two concentrated collinear forces on two opposite sides. The procedure is presented in detail because it can easily be extended to the calculation of critical stresses for rectangular plates with other boundary conditions and/or arbitrary loading. Since the given results can be considered "correct", they can be used as a basis for comparison with the results obtained by the finite element method or some other numerical method.