Explants of sympathetic ganglia from 3-9 day old rats were grown on collagen-coated coverslips in modified Rose chambers for 5 days, either alone or 2 mm away from explants of 6 week old rat caudal artery and aorta. Nerve fibre growth was stimulated on the side of the ganglion explant near the caudal artery explants but was not stimulated near the aorta. To determine the source of the nerve growth stimulation, explants of whole intact wall of the caudal artery, separated adventitia and medial layers, enzyme-dispersed cells, homogenized cells and medium pre-conditioned by caudal artery explants, were combined with ganglia. Explants of whole caudal artery and dispersed cells were also precultured prior to combination with ganglia. These combinations allowed analysis of the role of smooth muscle cells, existing nerve fibres, necrotic cells and connective tissue. Results suggested that degenerating nerve fibres within the blood vessels caused the increase in the number and the 'attraction' of the nerve fibres growing from the sympathetic ganglia. In contrast, both caudal artery and aorta from 3-9 day old rats caused stimulation of nerve fibre growth. Since these vessels were not yet innervated, the effect cannot be due to degenerating nerve terminals and a different mechanism must be involved.