These experiments present quantitative data concerning peripheral nerve regeneration in vivo. We used entubulation repair as a model to compare two different types of tubular prostheses, one nonbiodegradable and the other biodegradable. We modified the microenvironment of the regenerating axons within the tubular prostheses by adding a laminin-containing gel to the interior of the tube at the time of initial implantation. The data demonstrate that specific manipulations to the microenvironment of regenerating peripheral axons have quantitative effects on the rate and extent of nerve regeneration. Such effects were dependent on the composition of the tubular prosthesis and varied according to the survival time of the animals. For instance, the laminin gel within the biodegradable tubes enhanced nerve regeneration at 2 weeks but was inhibitory at 6 weeks. Furthermore, such manipulations may have different effects on the number of myelinated axons found within the regenerating nerve cable versus the number of primary motor and sensory neurons giving rise to such axons. We concluded that: the presence of a laminin-containing gel significantly increased the initial rate at which axons from primary sensory and motor neurons cross a transection site; an initial delay in axonal outgrowth at early time points did not necessarily predict diminished outgrowth at later times; and because of the potential for axonal branching the number of myelinated axons found in the midportion of a tubular prosthesis did not always correlate with the number of primary motor and sensory neurons which gave rise to those axons.