The effect of neonatal unilateral enucleations or combined enulcleations-visual cortex ablation on the neurons of the superior colliculus has been studied, Enucleation alone leads to a shrinkage of neurons in the upper portion of the stratum grieseum superficiale but does not affect those in the lower half. The ratio of asymmetric/symmetric synaptic terminals is decreased from 85/15 to 75/25. A small number of abnormal synapses is also found. When both eye and contralateral visual cortex are removed at birth, neurons throughout the stratum griseum superficiale are reduced in size 20-25%. The synaptic ratio is reduced further to 55/45, and about 15% of the post-synaptic structures encountered have abnormal presynaptic profiles attached. It is concluded that the extent of transneuronal atrophy and synaptic disruption is proportional to the amount of afferent input removed. Also, the synaptic modifications made in the upper layers of the superior colliculus appear to be local in nature and there is no evidence of a significant sprouting or new growth of the remaining axons.