The effects of neonatal capsaicin on the fibre populations of peripheral somatosensory nerves have been investigated in adult mouse sural nerve and rat saphenous nerve. One or two doses of capsaicin, 50 mg/kg, given in the first few days of life, caused a permanent 50% reduction in the number of unmyelinated axons in mouse sural nerve and a 64% reduction in rat saphenous nerve, compared with untreated controls. Schwann cell sub-units were also reduced in number, and on average contained fewer axons than controls. The number and size distribution of myelinated fibres were not affected by capsaicin. Active degeneration was seen in only a few unmyelinated axons, and a small number of denervated Schwann cell bands was present. There were no signs of unmyelinated fibre regeneration. It is concluded that neonatal capsaicin produces selective degeneration of unmyelinated axons in the neonatal period which has effectively reached its full extent by early adult life.