The Josephson plasma in RuSr2GdCu2O8 and FeSr2YCu2Oy is measured by the sphere resonance method. For ferromagnetic RuSr2GdCu2O8, the plasma is observed in a very low-frequency region (around 8.5 cm−1 at T≪Tc), which represents a large reduction in the Josephson coupling at ferromagnetic RuO2 block layers. For non-ferromagnetic FeSr2YCu2Oy, the plasma frequency increases to 31 cm−1, which is comparable to that of lightly doped YBa2Cu3O6+δ. The temperature dependence of the plasma does not shift to zero frequencies (i.e. jc=0) at low temperatures, indicating that there is no transition from the 0-phase to the π-phase in these compounds.