In the Rapsodie-I experiment, two bundles, each containing 34 fast breeder reactor fuel pins, have been irradiated up to a peak burnup of 10.6 at.% and to a peak fast fluence (E > 0.1 MeV) of 6.65 × 1026 n/m2. One of the main objectives of this experiment was to evaluate the mechanical behavior of a bundle with spacer grids. Two types of spacer grid designs have been tested: namely, a brazed ferrule grid design and a honeycomb spot-welded grid design. The grid material was in every case niobium-stabilized austenitic stainless steel type W.Nr. 1.4981 in the annealed condition. The density and the dimensional measurements carried out on the spacer grids revealed that the geometrical changes in the grids were almost entirely due to material void swelling. In some cases, however, mechanical interactions between grids and wrapper tubes and also between fuel pins and grid cells have been emphasized. These interactions had no detrimental influence on the in-pile bundle behavior. The postirradiation mechanical tests carried out on the honeycomb spacer grids showed that the mechanical properties of the grid cells have not been significantly altered by the irradiation. A decrease of the grid material Young’s modulus has been correlated with void swelling. It has been concluded that the spacer grids operated satisfactorily despite their severe loading conditions.