HAEM oxygenase-2 (HO-2) is the neuronal isoform of the only known mammalian enzyme generating the new transmitter candidate, carbon monoxide. Its distribution was investigated in the sympathetic nervous system. A 36 kDa HO-2 immunoreactive protein was identified in the particulate fraction of stellate ganglion and cerebellum in Western blots. Immunohistochemically, all noradrenergic and non-noradrenergic postganglionic neurones were HO-2 immunoreactive in prevertebral ganglia and cervical, thoracic, and lumbar sympathetic chain ganglia. Neither postganglionic nerve branches nor noradrenergic perivascular terminals were HO-2 immunoreactive under control conditions. However, accumulation of HO-2 immunoreactivity was found in noradrenergic axons in explanted sciatic nerves in which axonal transport was interrupted by crushing. We conclude that HO-2 is ubiquitous in perikarya of postganglionic sympathetic neurones, is subjected to axonal transport but does not accumulate at sympathetic neuroeffector junctions. This distribution favours a general role of HO-2 in sympathetic neuronal metabolism rather than a specific association with transmission at autonomic neuroeffector junctions.