Foreign body reaction of the liver, kidney, and subcutaneous tissue of rats after implantation of hollow fibers (HF) for 1, 3, 6, and 12 months and its influence on the effectiveness of diffusional transport of insulin and oxygen were investigated. The highest degree of fibrosis was observed after subcutaneous implantation of HF and the lowest degree after implantation into the kidney. Histochemical staining of the fibrous capsule showed a tissue-dependent domination of the collagenous fibrils. After 90 min of perfusion 33% of the insulin contained in HF diffused out of the nonimplanted fiber, after 120 min, 73% and after 180 min, 100%. Hollow fibers, which were removed with a surrounding connective capsule after an implantation period of 1 year, showed even better permeability for insulin than nonimplanted HF. The tension of oxygen in the lumen of the implanted hollow fibers was 42 mm Hg after implantation into the kidney and 30 mmHg after implantation into the liver. The oxygen present inside an HF that is implanted into a kidney and liver was consumed by 100 islets in 3.9 and 2.8 min, respectively. It was concluded that to achieve acceptable results in the construction of bioartificial pancreas more research activities should be performed on the diffusion and consumption of oxygen in the bioreactor.