Hydrogen embrittlement of AISI 4340 was studied using slow strain rate tests of both smooth and notched specimens. Two heat treatments of AISI 4340 were evaluated that produced yield strengths of 1479 MPa and 1140 MPa. The strain rate over the gauge section for the smooth tests and in the notch root for the notched tests was 1x10-6 /s. Testing environments included air at room temperature de-ionized water at 4°C and substitute ocean water at both room temperature and 4°C. Tests included specimens that had been precharged with hydrogen in 3.5% NaCl at 50°C for two weeks using a potentiostat to deliver a cathodic potential of -1100 mV to the materials. Slow strain rate tests in substitute ocean water were also accomplished at a cathodic potential of -1100 mV.Results indicated that the AISI 4340 was highly embrittled in SOW. The higher strength material showed much more embrittlement. Testing of the AISI 4340 at both 20°C and 4°C showed that the temperature had no noticeable effect on the response. These results will be related to analyses of the fracture surfaces using SEM and optical microscopy.