Fatigue tests were conducted in sodium to investigate the influence of an elevated temperature sodium environment on the low cycle fatigue behavior of Types 304 and 316 stainless steel The fatigue tests were conducted at 550°C in sodium of controlled purity, namely, 1 ppm oxygen and 0.3 ppm carbon, at a strain rate of 4 × 10−3s−1. The fatigue life of annealed Type 316 stainless steel was substantially greater when tested in sodium than when tested in air, whereas the test environment had little influence on the fatigue life of Type 304 stainless steel. The effects of long-term sodium preexposure were also investigated. An 18-Ms (5000-h) preexposure to sodium produced little effect on the fatigue life of Type 316 stainless steel tested in sodium. However, a similar sodium exposure had a significant effect on the fatigue behavior of Type 304 stainless steel.