The effects of monocular deprivation, beginning at hatching, were examined in the neuropil of ectostriatum, a visual telencephalic projection area in birds. The volume of ectostriatum, the number of synapses and subsynaptic features like the presynaptic terminal size and the length of the postsynaptic contact zone were quantified in juvenile (20 d) and adult (100 d) zebra finches. Monocular deprivation affects almost all of the parameters mentioned above in juvenile birds, but only one (i.e., the size of presynaptic terminals) is permanently altered in adulthood. Both hemispheres are affected in juvenile birds with respect to the volume of ectostriatum, the length of synaptic contact zones and the presynaptic terminal size when compared to normal values. In normal birds the number of synaptic contacts is established at 20 days and remains fairly constant at 100 days. In experimental birds there is an increase in synapse number during this time period. However, no interhemispheric differences or differences compared to normal animals could be identified. The presynaptic terminals in experimental birds are smaller compared to normal values in young (25% for the deprived side; 19% for the non-deprived side) and adult (13% for the deprived side; 11% for the non-deprived side) animals. The only permanent effect caused by monocular deprivation in the ectostriatum is characterized by smaller presynaptic terminals. It is surprising that the tectofugal pathway that is believed to be mostly ipsilateral is not very vulnerable to monocular deprivation in adult animals. It is even more surprising that the deprivation effects are seen on both sides of the brain. The implications of this plasticity will be discussed in this paper.