We have examined an identified serotonergic neurone in Achatina fulica and described the normal morphological and physiological characteristics of this cell. Injury-induced changes in this neurone following in vivo recovery are described and compared with in vitro gastropod models of regeneration. Nickel-lysine and biocytin dye-fills of the metacerebral giant (MCG) neurone, together with serotonin-like immunoreactivity, revealed an extensive innervation of the ipsilateral buccal ganglion, much greater than that previously reported. Labelled MCG fibres were seen to ramify throughout the ganglion, providing extensive neuropilar innervation. Serotonin-immunoreactive fibres were seen not only within the neuropile but also within the cell body layer of the buccal ganglia, surrounding many of the cell bodies with varicose fibres. Dye-fills also revealed a minor contralateral buccal innervation not previously described. This view of a predominantly ipsilateral innervation of the buccal ganglia by the MCG was supported by electrophysiological measurements. The ipsilateral buccal follower cell B1 displayed an increase in depolarization in response to repeated trains of action potentials to the MCG, whereas the contralateral B1 showed only a weak depolarization in response to the identical stimuli. Following a crush to the cerebral-buccal connective (CBC), the MCG rapidly regenerated its injured projections, displaying both morphological and physiological recovery within 5-10 days. The original, severed fibres of the MCG were, however, replaced by a multitude of smaller neurites, which persisted for up to 3 months (the longest recovery period examined). Despite this morphological difference between normal and regenerated fibres, the MCG re-established functionally equivalent connections upon B1. In contrast with previous in vitro studies using gastropods, serotonin-like immunoreactivity revealed that severed distal fibres from the MCG rapidly degenerated (2-6 days), resulting in a transient unilateral depletion of serotonin in the buccal ganglia. We suggest that this loss of serotonin in the lesioned ganglion may play a functional role in regeneration, as has been suggested in vitro.