The morphology and projections of ventral horn interneurones in the segment above an ipsilateral thoracic lateral spinal cord lesion were studied in the cat by intracellular injections of Neurobiotin at 6 to 18 weeks post-lesion and compared with previously published control data from uninjured spinal cords. The cell axons ascended, descended or both, mostly contralaterally and mostly spared by the lesion. Unusual morphological dendritic features were seen in the lesion group, mostly growth-related, including complex dendritic appendages, twisted or multiple-branched terminal dendrites, commissural dendrites, apparently swollen proximal dendrites and rostrocaudal asymmetries. Significant quantitative differences included more dendritic spines in the lesion group (3.4 x) and smaller soma areas in the lesion group (with similar numbers of primary dendrites and rostrocaudal dendritic spans). Immunoreactivity to microtubule associated protein 2a/b was detected in the proximal, but not distal, dendrites of cells in the lesion group, corresponding to an overall decrease in immunoreactivity in the ventral horns on the lesion side compared to the other. For axon collaterals, significant increases for the lesion group were seen in the number of collaterals in the first 4 mm of axon and in the area of ventral/intermediate horn occupied by terminals, including increased innervation of some regions, among which were the intermediolateral columns. This dendritic and axonal plasticity makes the interneuones candidates for a role in detour circuits but also for a maladaptive role in autonomic hyperreflexia.