Isolated adult frog dorsal root ganglion neurons survive in vitro in a defined medium for more than 4 weeks and extend processes. When co-cultured with a 1-mm piece of peripheral nerve the average total process length per neuron was 10 times longer than that of control neurons by 8 days, and the processes had a significantly different morphology from that of control neurons. This influence on process length increased with increasing time of nerve denervation prior to co-culturing. These results suggest the release of a neurotrophic factor/s from the cells of the peripheral nerve. The neurotrophic influence was completely blocked by antibodies against mouse nerve growth factor (NGF). Although NGF increased the average process length by twofold over control neurons, its influence nerve reached that of the nerve-released factor, and the NGF-induced processes had a distinctly different morphology. The frog nerve-released factor promoted process outgrowth from E11 chick sympathetic ganglia, although the process number, length, and their fasciculation differed greatly from those induced by NGF. These results suggest that the nerve-released factor/s are immunologically and functionally related to NGF but have not established whether a single factor or an aggregate of several secreted molecules are responsible. This article presents a new preparation in which the varied influences of different neurotrophic factors can be studied in great detail on large populations of isolated adult vertebrate neurons and sets the stage for the characterization and isolation of the frog peripheral nerve neurotrophic factor, as well as examining the influence of this factor on neuronal morphology and its ability to direct process outgrowth.