Microelectromechanical systems (MEMS) are used to machine miniaturized implantable medical devices. Our group has used MEMS technology to develop hemofiltration membranes for use in renal replacement therapy, which possess enhanced selectivity and permeability. The use of silicon in blood-contacting environments may be limited, however, due to contact activation of the coagulation cascade by silicon, which forms the surface oxides in atmospheric conditions. As well, the reports of long-term biocompatibility of blood- contacting silicon devices are lacking. The aims of this pilot study were as follows: 1) to develop a model for investigating the effects of intravascular implants and 2) to characterize the degree of thrombosis and tissue inflammation incited by prolonged implantation of silicon materials. Silicon implants with and without polyethylene glycol (PEG) coatings were surgically implanted transluminally through rat femoral veins. Gore-Tex and stainless steel implants served as controls. The implants were left in vivo for 4 weeks. All femoral veins remained patent. The veins associated with silicon implants exhibited rare thrombi and occasional mild perivascular inflammation. In contrast, Gore-Tex and stainless steel controls caused moderate vein thrombosis and provoked a moderate to marked cellular infiltrate. Under scanning electron microscopy, bare silicon implants were found to have significant adherent microthrombi, whereas PEG-treated implants showed no evidence of thrombi. PEG-treated silicon seems to be biocompatible and holds potential as an excellent material with which to construct an implantable, miniaturized hemofiltration membrane.