Fatigue testing was carried out primarily on a medium strength low alloy steel manufactured for use in sour environments. The testing was done in a sour brine solution that simulated drilling fluid. Test variables included hydrogen sulfide overpressure, stress ratio, fatigue test frequency and test temperature. Sour fatigue thresholds were increased compared to non-sour conditions, particularly as the fatigue cycling frequency was decreased, as the stress ratio was decreased and as the testing temperature was increased. The increases correlated with crack closure effects from the corrosion products. Implications for operations in sour environments are discussed. Keywords: fatigue, sour brine, corrosion fatigue, fatigue threshold, hydrogen sulfide, steel.