Implantable biomedical sensors built on a silicon substrate capped with glass are currently being developed for intravascular applications. Electrical techniques for inhibiting thrombus formation on the surface of a proposed optical sensor in direct contact with blood have been investigated. Glass-on-silicon specimens (4 X 1.2 X 0.4 mm3) were coated with indium-tin oxide, a transparent conductor, and implanted in the vena cava and iliac veins of three dogs for 10, 20, or 33 days. The equilibrium surface-blood interface potentials of the specimens were modified by implanted current sources which supplied either direct current (8-15 microA) or 100 KHz alternating current (5 microA, root mean square). Light-microscopic and scanning electron-microscopic analyses showed each of the DC-polarized specimens to be free of thrombus, in contrast to nonpolarized (control) specimens on which varying amounts of adsorbed protein and thrombus deposits were found. Like the control specimens, the AC-polarized specimens formed thrombus, but the appearance of the deposits differed. These findings support the view that the polarity, magnitude and time dependence of the potential across conducting surface-blood interface significantly influence thrombogenicity. Further work is necessary to determine the roles of electrochemical and electrostatic factors in preventing thrombus formation on foreign materials.