An exogenous fibronectin-laminin (FN-LAM) solution was added into silicone chambers to determine the effects on peripheral nerve regeneration across 18-mm long gaps. The sciatic nerves of adult rats were sutured into silicone chambers 20 mm in length, creating an 18-mm gap between the proximal and distal nerve stumps. The chambers were filled with either a mixture of fibronectin and laminin (500 μg/ml each) or a solution of cytochrome C (1 mg/ml) as the control. After six weeks, the animals were killed and the chambers were examined for regeneration. Seventy percent of the animals from the FN-LAM group demonstrated regeneration across the 18-mm gaps, compared to only 30% in the control group. The combination of FN-LAM significantly increased the number of axons that grew into the distal end of the chamber (FN-LAM, 1325 ± 522; cytochrome C, 153 ± 104; p = 0.03). Examination of the distal tributaries of the sciatic nerve revealed axons only in the FN-LAM group; none were found in the control group. Quantitative analysis of neurons, retrogradely labeled with horseradish peroxidase via injection of the sciatic nerve distal to the regenerated segment, revealed a greater number of sensory and motor neurons in the FN-LAM group compared to the control group. Morphometric studies revealed that the mean area of the regenerated segment in the FN-LAM group was 37% larger than the controls, and ultrastructural analysis demonstrated a more mature regenerated nerve. This is the first in vivo demonstration that this combination of fibronectin and laminin significantly enhances the regeneration of myelinated axons across a long nerve gap in the rat sciatic nerve.