Naturally occurring and lesion induced neuronal death have been studied in the developing rat thalamus. Natural cell death was present from embryonic day (E)19 until postnatal day (P)8 with a peak occurring at birth. Counts of total neurones indicated a postnatal loss of 27%. Unilateral section of the infraorbital nerve at birth was associated with increased cell death in the contralateral thalamus; this was maximal at P2 and continued until P10. Counts of neurone numbers showed a reduction of about 24% of neurones on the contralateral side. A smaller, more transient decrease was seen ipsilaterally. The lesion induced cell loss was associated with a decrease in volume of the ventrobasal complex, with minimal reduction in cell density.