Clinical studies have shown significant release of metal bearing ionic species from total joint replacements, both acutely and in the long term. In vitro cytotoxicity and animal implantation studies suggest adverse biological roles for such released materials. However, their origin is unclear: they may be produced by uniform corrosion, or by wear and fretting during articulation or by some combination of these and other processes. A series of in vitro and in vivo studies is proposed to examine some features of metal release in the two common alloy systems in use today: cobalt-base and titanium-base. Four main experimental models are to be used: in vitro passive release, anodic release and cyclic loading and release from an in vivo rat THR. Independent experimental material parameters will include alloy composition, heat treatment and surface treatment. Analytical techniques will include atomic absorption spectroscopy (AAS), scanning electron microscopy with electron diffraction x- ray analysis (SEM/EDAX) and secondary ion mass spectroscopy (SIMS).