The present experiments were designed to determine whether the cells of the distal portion of a sectioned peripheral nerve release diffusible factors that exert a trophic influence on isolated adult motoneurons in vitro, whether the release of those factors increased with time following nerve section, and to characterize the neurotrophic influences. In defined medium, motoneurons started to extend processes only after 1 day. However, when co-cultured with a length of sciatic nerve, denervated one week prior to being placed in the co-cultures, the motoneurons began to extend processes within one day, and by four days the average process length was 10 times longer than that of control neurons. While control neurons extended 4 processes, the co-cultured neurons extended only a single process, and the processes were thinner and straighter than those of the control neurons, and they lacked lamellipodia, typical of control neurons. Although one-week predenervated peripheral nerve initially had a greater influence on motoneuron process outgrowth than a freshly isolated piece of sciatic nerve--after three days the processes were 33% longer--, this difference decreased with time, so that by seven days this difference was only 7%. Thus time was required for the release of the neurotrophic factor following nerve section. Both predenervated and freshly sectioned sciatic nerves had the same influences on the number and morphology of processes of the co-cultured motoneurons. The neurotrophic influence of medium conditioned for seven days by a piece of sciatic nerve was the same as that seen when the cells of the peripheral nerve were in the medium. Nerve growth factor did not induce process elongation from the motoneurons. In addition, antibodies against nerve growth factor, known to block its bioactivity, did not diminish the neurotrophic influence of the sciatic nerve conditioned medium on the motoneurons. These results indicate that factors released from the cells of a sectioned peripheral nerve have a potent neurotrophic influence on adult motoneurons, the release of these factors increases with time following nerve section, and that the factor does not appear to be nerve growth factor. These observations suggest that the cells of the denervated distal nerve play an important physiological role in vivo, in releasing diffusible factors that act on motoneurons to promote significant process outgrowth, regulate the number of processes extended, and modify the morphology of the processes, leading to reinnervation of the distal nerve stump.