Torsional fatigue tests were conducted on 4370 steel oil quenched and tempered at 1000° F in (l) the uncharged state, (2) the hydrogen charged state, and (3) in a hydrogen environment. The tests were conducted on both smooth (K_t = l.l) and V-notch (K_t = 3.8) test specimens. A statistical analysis conducted at the 99% confidence limit for the smooth test specimens indicated that precharging with hydrogen increased the fatigue life of the material and also the fracture surface of the test specimens changed from a circumferential crack to a 45° diagonal crack. At a 90% confidence limit, charging with hydrogen did not affect the fatigue life of the V-notched specimens. At a 99% confidence limit for both the smooth and V-notched test specimens, testing in a hydrogen environment increased the fatigue life of the material. Bending fatigue tests were also conducted on the same material and the results indicated that charging with hydrogen decreased fatigue life of smooth test specimens (K_t = l.l) but increased the fatigue life for V-notched specimens (K_t = 2.4 and 3.8). / Ph. D.