INTRODUCTION This note describes the method of analysis and computation procedure used in Item No. 90016 (Reference 1) for the estimation of natural frequencies of flat rectangular plates. The method of analysis is applicable to the computation of the lower natural frequencies@ and mode shapes@ of isotropic or specially orthotropic plates. Plates are specially orthotropic when their principal axes of orthotropy are parallel to the plate edges. The method of analysis is based on that described in Reference 2 and uses the Rayleigh-Ritz method in which the displacements are assumed to be represented by a series of orthogonal polynomial functions. The polynomial functions are chosen to satisfy the plate boundary conditions. In the analysis@ provision is made for constant in-plane forces@ acting on the middle surface of the plate@ in both the x- and y- directions together with a constant in-plane shear force.