The effects of different periods of monocular deprivation on cell sizes in the ectostriatum, the telencephalic relay of the tectofugal pathway in zebra finches, were evaluated. Following 20 days of monocular closure, neurons in the deprived and undeprived hemisphere show an unselective hypertrophy of 10%. Extending the deprivation period results in a shrinkage of neurons of the deprived side to values of adult normally reared birds, whereas the non-deprived neurons maintain their hypertrophied size.