The goal of this study was to determine the effect of assembly load and local assembly environmental conditions on the fretting corrosion of modular femoral stem tapers. Femoral head/ taper assemblies in both similar (CoCrMo/CoCrMo) and mixed (CoCrMo/Ti-6Al-4V) alloy combinations were evaluated using an electrochemical test method. Specimens were assembled under impact loading and by hand, in both wet and dry conditions. Incremental cyclic loads ranging from 89 to 5,340 N were applied at a frequency of 3 Hz in Ringer's solution at ambient temperature. During the test, both the open circuit potential (OCP) and fretting current (i fret ) were measured using a saturated calomel electrode (SCE) and counter electrode, respectively. The results were comparable for both mixed and similar alloy couples. Decreases in OCP and increases in i fret (indicators of oxide film fracture and repassivation) were seen with increasing load magnitude, often occurring at loads well below those expected clinically. OCP at the 5,340 N cyclic load ranged from À30.4 to À103.7 mV versus SCE for similar alloy couples, and À19.1 to À181.4 mV versus SCE for mixed alloy couples. Mean peak fretting currents ranged from 0.84 to 1.42 mA and 1.06 to 3.12 mA for similar and mixed alloy couples, respectively. The larger current magnitudes and more negative shifts in OCP for mixed alloy couples indicate the difference in oxide film fracture behavior between titanium and cobalt alloys. The load at which OCP began to drop (onset of fretting) was dependent upon the assembly conditions for both material couples. Specimens assembled with impact loads in air showed the highest resistance to fretting. The results of this study indicate that the assembly load and the environment both play a role in the initial stability of modular hip taper connections.