Bridging large defects in mixed nerves is still an unsolved problem in reconstructive microsurgery. Two main aspects may be distinguished: one is to obtain an appropriate substitute for the lost neural tissue, the second to direct fibers toward their previous end-organs with the highest possible specificity. In the present study, sural nerve block grafts were combined with enclosed gaps at one or both ends of the grafts. Functional outcome at the muscle level as well as the number of motor axons and their cross-sectional distribution were assessed after 3 months. The presence of a proximally placed tube was found to decrease significantly the maximal tetanic force of the tibialis anterior muscle, whereas a distally placed one tended to improve it. Morphological data from acetylcholinesterase histochemistry correlated poorly with functional results but they gave some clues about possible roles played by the chambers, according to their position relatively to the grafts. No definitive evidence for an improved regeneration by use of silicone tubes in addition to the conventional grafts could be demonstrated.