164 inbred Wistar rats received subcutaneous and intraperitoneal implants of xenogeneic collagen sponges and were examined by light- and electron microscopy as well as immunohistologically, to establish tissue and cell reactions and the time of collagen reabsorption. 4 to 5 weeks after implantation the xenogeneic collagen sponge was completely absorbed. All the inflammatory and reparative reactions were non-specific. Granulation tissue appeared soon (3 days after implantation) with new synthesis of fine collagen fibrils, mainly of type III, followed by type I collagen fibers. Myofibroblasts, with intracellular evidence of myosin and collagen (Type I and III) were sporadically present in the granulation tissue after day 5 and regularly after day 10. Small scar granulomas without xenogenic collagen were detectable even 4 to 6 months after collagen implantation. Reabsorption and regression of collagen granulomas occurred faster after xenogeneic collagen implantation than after using surgical cat sutures. From the morphological point of view, there is no indication of specific sensitization against xenogeneic collagen or of a specific immunologic process.