The infraorbital nerve was cut in either neonatal (on day 0) or adult (day 60) rats and the peripheral regeneration prevented. After 60 days either anatomical or electrophysiological techniques were used to study the peripheral nerve, trigeminal nucleus and somatosensory cortex. In neonatally sectioned animals the number of myelinated fibres surviving, at 60 days, in the peripheral nerve proximal to the lesion was 11% compared with 100% survival after adult nerve section. This reduction in surviving nerve fibres in neonatally lesioned animals was associated with a significant reduction in cross-sectional area of all trigeminal nuclei (principalis, oralis, interpolaris and caudalis) of 18-29%. No significant change in area was present in animals sectioned as adults. Neonatally lesioned animals also showed a reduction of approximately 20% in the number of cells visible in cross-sections of all trigeminal nuclei. Animals sectioned as neonates showed marked plasticity at all nuclei in the trigeminal complex as well as in the cortex. Deafferented cells responded to new peripheral receptive fields so that the somatotopic organization of these cells was modified. Such cells are referred to throughout as 'reactivated' cells. However, in animals sectioned as adults no evidence of plasticity could be detected in the trigeminal nuclei. Only very limited reactivation was apparent in the cortex, so that the majority of deafferented cells remained unresponsive at both sites. A detailed comparison was made of twenty-three reactivated cells and twenty-five normal cells from nucleus principalis of animals with nerve section on day 0. The reactivated cells commonly showed larger, more complex receptive fields, longer latencies and lower following frequencies, although stimulus thresholds were similar. Thus reactivated cells showed more convergence and poorer synaptic security than normal cells. However, stimulation of the contralateral thalamus produced similar responses from both groups of cells, suggesting that not all inputs to reactivated cells were modified. The time course of the reactivation of cells in nucleus caudalis from animals lesioned on day 0 was followed over 30 days. No acute effect, for up to 24 h, was detected. However, somatotopic reorganization had started by day 7, proceeded rapidly between days 7 and 14, and was completed by day 21.