Matrix formation within silicone tubes of different geometries implanted between the stumps of the transected rat sciatic nerve was studied. A matrix, composed of longitudinally oriented fibrin strands containing fibronectin, was formed within one day. The matrix then increased in size. The matrix contained macrophages and other inflammatory cells. Matrix size could be increased if the diameter of the tube was increased or if holes were made in the implanted tube. In contrast, matrix diameter decreased if the tube length was increased or if circulation was compromised in the inserts. The results suggest that the size, orientation and cellular components of the matrix have profound effects on the regenerative response of the transected nerve.