Abstract The RKKY interaction in metallic Eu xGd 1−xB 6 compounds has been studied by EPR technique for x = 0.00, 0.20, 0.40 and 0.50. In the region of the absorption maximum the experimental spectra can be fitted to a lineshape function consisting of both Lorentzian absorption and dispersion components. Weiss constants, exchange fields, unbottlenecked g-shifts, and thermal broadening are calculated on the basis of the RKKY model and compared to the experimental results. The predicted Weiss constants and exchange fields are in good agreement with experiment, indicating the validity of the RKKY model. However, the predicted g-shifts and thermal broadening are much greater than those measured experimentally. The reduced g-shifts and thermal broadening are interpreted in terms of a magnetic resonance bottleneck, and the relaxation rates due to the interaction of the conduction electrons with the localized moments and lattice are evaluated.