We describe an experimental model to study regeneration of lesioned corticospinal tract (CST) fibers in the adult rat spinal cord. After transection of all CST fibers at mid-thoracic level the gap is grafted with a sterile, cell-free collagen matrix. Two methods of collagen-application are used: 1) injection of a fluid collagen solution into the lesioned area which self-assembles in situ and 2) implantation of a solid collagen gel. At 4 weeks post-implantation CST axons are anterogradely labelled with horseradish-peroxidase (HRP). The collagen implant is evaluated for ingrowth of CST axons. The histopathological reaction (gliotic response) around the lesion and within the matrix is also studied. After application of a fluid collagen solution into the lesion area HRP-labelled CST axons can be visualized within the implant. In addition, astroglial and reactive microglial cells invade the collagen-matrix. On the other hand, if collagen is implanted as an already self-assembled gel, no ingrowth of labelled CST axons nor of astroglial/reactive microglial cells is observed. Both methods of collagen-application result in a considerable reduction of the gliotic response as compared to the ungrafted animals. We conclude that the method of application of collagen (i.e., fluid or gel) considerably affects the response of lesioned CST axons. The application of a fluid collagen graft which in situ self-assembles is beneficial for the regrowth of lesioned CST axons in rat spinal cord. In this respect the formation of an astroglial scaffolding structure within the (fluid) collagen, probably due to optimal integration between host and graft, is very important. The inability of injured CST fibers to enter the solid collagen graft may be related to the absence of an astroglial scaffolding structure within the implant.