During development of the central nervous system neuronal growth cones are attracted or repelled by concentration gradients of specific target-derived factors. Although evidence suggests that gradients of target-derived factors are chemotropic for adult motoneuron growth cones, a conclusive demonstration has remained elusive. Further, no preparation has been available on which to rapidly and reliably test candidate factors. The present results demonstrate that in vitro adult frog motoneuron growth cones consistently turn and grow up diffusible concentration gradients of denervated peripheral nerve-released factors over distances of almost 200 microm, and that the responsible factor/s are larger than 30 kDa. This preparation provides an excellent bioassay on which the factor/s responsible for this influence can be tested as they are isolated and characterized.