A key role has not yet been identified for beta nerve growth factor (NGF) in the growth responses that continue to be expressed in the sensory neurons of adult animals. We have now examined the effects of daily administration to adult rats (and in a few experiments, mice) of antiserum to NGF on (i) the collateral sprouting of undamaged nociceptive nerves that occurs into denervated adjacent skin and (ii) the regeneration of cutaneous sensory axons that occurs after they are damaged. The results were unexpected. All collateral sprouting was prevented and that already in progress was halted; sprouting resumed when treatment was discontinued. In contrast, the reestablishment, and even enlargement, of cutaneous nerve fields by regenerating axons was unaffected by anti-NGF treatment, even after dorsal rhizotomy was done to eliminate any central trophic support. In denervated skin, regenerating and collaterally sprouting axons utilized the same cellular pathways to establish functionally identical fields, thus displaying apparently identical growth behaviors, yet anti-NGF treatment clearly distinguished between them. We suggest that endogenous NGF is responsible for the collateral sprouting of nociceptive axons, probably reflecting an ongoing function of NGF in the regulation of their fields. This demonstration in the adult sensory system of a defined role for NGF in nerve growth could apply to nerve growth factors generally in the adult nervous system. The regeneration, however, of nociceptive axons (and nonnociceptive one) is not dependent on NGF.