Summary The processes of degeneration and reinnervation in the optic tectum of Xenopus laevis have been detailed by quantitative ultrastructural analysis. The tecta were denervated either permanently, by removing the contralateral eye, or temporarily, by cutting the optic tract. After denervation the total number of synapses decreases rapidly within 4 days to a stable level of about 40% of the original number. Vacated postsynaptic sites are subsequently removed by phagocytosis. When regenerating axons arrive in the tectum they make synaptic contact by inducing new postsynaptic membrane specializations. The ultrastructural sequence of reinnervation by optic axons resembles initial synaptogenesis.