A light microscope study has been made of the uptake and transport of horseradish peroxidase (HRP) by axons of intact and damaged peripheral nerve trunks. Topically applied HRP entered axons of guinea pig cervical vagi and hypoglossal nerves whether or not these were damaged by crushing or local freezing at the site of application of HRP. If damaged, the corresponding cell bodies contained HRP, but little was detected in the cell bodies of undamaged axons. If the damage was distal to the site of HRP application, there was no significant retrograde transport. The presence of HRP within axons appears to be insufficient for its transport; traffic is initiated only from axon terminals and regions which have been damaged.