The sizes and histological differentiation of structures in the central nervous system of wild and domestic ducks were compared using allometric methods. Whole brain volume is 14.3% less in domestic ducks than in wild birds, and the size of certain brain structures is more variable in domestic ducks than in the wild birds. These findings are consistent with results of studies in other domestic birds and in domestic mammals. Although the brain subdivision volumes are not reduced to the same extent, the relative composition of the brain is maintained in domestic ducks. The overall reduction in rhombencephalic parts of the brain (16.3%) is greater than that in the prosencephalon (13.5%). Within sensory centers the nuclei of the trigeminal system undergo the most marked decrease (21.3%). Optic (13.3%) and limbic (11.1%) structures are less reduced, and the olfactory regions do not show any differences in volume compared to these regions in wild mallards. Reductions in the volumes of brain structures in domesticated animals have been thought to be processes of functional adaptation controlled by artificial selection. Our observations are discussed with regard to hypotheses about trends in domestication and evolutionary plasticity.