We have investigated far-infrared excitations in rectangular arrays of antidots in modulation-doped AlxGa1-xAs-GaAs heterostructures. In the rectangular antidot lattice the degeneracy of modes of a square array is lifted and a rich spectrum of higher-frequency modes is excited, particularly at small magnetic fields. These modes are sensitive to the direction of linear polarized radiation. Our experimental results are compared with theoretical calculations based on the Thomas-Fermi-Dirac-von Weizsäcker hydrodynamic theory.