The numbers of sympathetic preganglionic neurones in the upper thoracic spinal cord retrogradely labelled after application of horseradish peroxidase to the severed axons of the cervical sympathetic trunk have been determined in guinea-pigs and in cats. Neuronal counts were made on both sides of spinal segments C8 to T10 in control animals and in others in which the enzyme was applied 5-8 days after the left cervical sympathetic trunk had been transected. Labelling was not significantly different between sides of unoperated animals, but after previous cervical trunk section labelling was always less, by from 5 to 100% of that on the control side. The results were not significantly different if the application of horseradish peroxidase was made close to the original lesion or 1 cm more proximal along the nerve trunk. Analysis of the dimensions of labelled cells suggested that the deficit in labelling was greatest for small preganglionic neurones. These findings do not support an earlier report14 that retrograde labelling from regenerating axons is enhanced at this stage after axotomy, and the possible reasons for the discrepancy in results are discussed. It is suggested that labelling of cell bodies may not be restricted if intact axon sprouts are exposed to horseradish peroxidase. Reduced labelling from axons at the time of a second transection might, at least in part, be due to axon atrophy, emphasizing limitations to labelling when horseradish peroxidase is applied to severed axons of fine diameter. The high numbers and reproducibility of our control data enabled estimates to be made of the segmental distribution of the cell bodies of origin of the axons of the cervical sympathetic trunk in both species. © 1984.