Wallerian degeneration is very slow in the mouse strain now known as C57BL/Ola. Sensory axon regrowth following peripheral nerve lesions is very poor in these animals but motor axons succeed in reinnervating the distal nerve stump even while the majority of severed axons are still intact (Lunn et al., Eur. J. Neurosci., 1, 27 - 33, 1989). To see if motor axons could grow into a completely undegenerated portion of nerve, the proximal stumps of the peroneal and tibial nerves were sutured together in six BALB/c mice and the ability of large motor and sensory fibres from the tibial nerve to grow into the peroneal nerve was examined electrophysiologically in four of them. For the acute experiment the peroneal nerve was cut approximately 7 mm central to the point of suture to the tibial nerve. Both at 2 weeks and 7 weeks after surgery the size of the potential recorded in the ventral roots on stimulating the portion of peroneal nerve into which tibial axons were directed to grow was only approximately 8% of the potential recorded when the tibial nerve was itself stimulated. The potential recorded in the dorsal roots was only approximately 2%. Counts of axon numbers in electron micrographs showed a small but non-significant increase over normal in the number of unmyelinated axons in the peroneal nerves which had been connected to the tibial nerve in this way. It is concluded, in agreement with Langley and Anderson (J. Physiol., 31, 365 - 391, 1904), that axon growth into intact nerves is extremely limited in mammals and that the distal nerve stump of C57BL/Ola mice, although it degenerates very slowly, is not therefore equivalent to an intact peripheral nerve.