Helium concentrations have been measured in sections of Type-304 stainless-steel control and safety rod thimbles irradiated in EBR-II to a peak fluence of 8.8 × 1022 n/cm2. The results, obtained by high sensitivity gas mass spectrometric techniques, show that more helium is produced than is predicted from present calculations especially at the higher temperature regions of the rods. It is concluded that sources of helium by the (n,α) process with elements other than the primary constituents of stainless steel contribute a significant fraction of the total helium produced and that one or more of these impurities might be migrating to the hotter surfaces of the stainless steel. A 45% gradient of nitrogen concentration along one rod was measured but that alone seems insufficient in magnitude to explain the helium gradient.