Polyether polyurethanes are extremely interesting for use in implantable devices. They are, however, susceptible to autoxidative degradation and stress cracking. One approach to improving biostability is to replace some of the polyether with polysiloxane. Shore 80A polyether polyurethanes with 20% (PS-20) and 35% (PS-35) polysiloxane were strained to 400% elongation and implanted in rabbits. Twelve weeks implant showed that both were significantly more biostable than their polysiloxane-free controls. After 18 months implant, PS-20 developed some localized tensile fractures. PS-35 showed no sign of visual damage. Infrared surface analysis does not allow direct evaluation of autoxidation because the Si--O--Si stretch peaks mask the polyether bands. Secondary indicators suggest possible very slight autoxidation of both PS-20 and PS-35 surfaces, but not enough to develop cracks. The polysiloxane-free controls did show substantial infrared evidence of autoxidation. Molecular weights of long-term PS-20 and PS-35 explants were negligibly lower. In comparison, the polysiloxane-free control suffered 35% molecular weight loss. Positive and negative controls performed as expected. PS-20 is recommended for devices that do not sustain high fixed loads. PS-35 is dramatically more biostable than its unmodified polyether analogues and is recommended for use in chronically implantable devices.